My goal is to get 25+ mpg from my 99 XJ with 30 x 9.5 R15 ATs -
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Unread 07-21-2013, 01:32 PM   #1
Web Wheeler
1999 XJ Cherokee 
Join Date: Mar 2013
Posts: 4,352
My goal is 25+ mpg in my 99 XJ with 30 x 9.5 R15 ATs, 3.55 diff gears, auto tranny

Preface This preface was written after this thread existed.
This OP has been revised numerous times as I gained new information and more experience (and will be revised again as necessary). This OP cites specific posts (including post #s) that have more details about specific sub-topics. The idea is that this OP can serve as a table of contents for the most important sub-topics. When possible, and appropriate, I cite the specific parts brands and part #s I used.

I am not a mechanic. I am an enthusiastic amateur with 19 years experience on three Jeeps with 4L engines. I experimented with improving gas mileage on each Jeep, and I've gotten more successful with each Jeep as I learn new tricks. I have done many things (documented in this thread) on my current Jeep (99 XJ) as an experiment and hobby to increase fuel economy as much as possible with a modest mod budget. I'm sharing this info so that it may be of use to others, or at least interesting to some.

In addition to my ideas there are many excellent ideas contributed by others in this thread. Many of the ideas have been implemented successfully, some haven't been implemented, and one failed but was a good learning experience. The details are are documented in this thread. This thread is a successful community project and is ongoing. I appreciate the economodding ideas and advice of others and I sometimes implement their ideas.

If you decide to participate and try some of these mods, you will be participating in a fun project that might benefit your gas mileage, but I cannot promise specific results because your Jeep is different that mine with more or less miles and wear on it, more of less lift, tire size, or weight, etc.

My gas mileage has gone from 15 to 25 mpg at 60 mph on level highway with $800 cost in parts, and I'm not finished yet. I still have more mods I want to do, but I have done most of the low cost mods already. If I do much more mods it will begin to get expensive. I am currently saving $96 a month in gas (after spending $800 on parts). It took me 2 years of experimentation with my current Jeep to get to this point.
I'm not counting the cost of parts for my failed experiments. I'm only counting the parts costs of my successful modifications that proved to increase mpg and/or performance in experiments-tests. Some of my successful mods improved mpg without loss of performance (power/torque), some improved performance without loss of mpg, and some improved BOTH mpg and performance. My Jeep now has more mpg and more performance! I also had some failed mods that didn't help mpg or power, or and one failed mod that diminished mpg and torque. I'll only steer you towards the mods that were successful for me.
There was trial and error and experimentation involved to figure out which mods help mpg (and how much), what doesn't help, and which hurts mpg. I used my common sense, some basic scientific knowledge, and various Internet forums for ideas and suggestions mods to try. Some ideas/mods worked and some didn't. Experimenting on my Jeep was the only way to find out which mods were good. The majority of my ideas and others' ideas worked successfully to varying extents, but some ideas didn't change mpg or performance at all, and some made mpg or performance worse. Hence the experimentation to find out what really works at an affordable cost. The beauty of this thread is myself and other participants in this thread have done a lot of experimentation to find out and document what works and what doesn't. This saves you some trouble, time, and cost of guessing and experimenting. You can implement our successful ideas and omit our failed ideas.
This thread can help you short-cut the process by following my recipe so you won't have to experiment much. However, the project should still be viewed as your experiment, not an investment, because exact results cannot be guaranteed. It only required $400 parts to get my XJ from 15 to 23 mpg. It required an additional $400 parts to get from 23 to 25 mpg. That's a total of $800 parts to get to 25 mpg. Not bad at all, especially since around $100 of that was normal tune-up parts that were needed anyway. I estimate an additional $1000 parts would probably get me from 25 mpg to 27 or 28 mpg, but is it really worth it? If I decide it is worth it, I have a good idea what additional mods I'd do. But the main focus of this thread isn't a high dollar build. It's about helping the average guy get substantially improved gas mileage with as good or better performance using a modest parts budget of under $1000.

Keep in mind that I have a lift, 30" tires, and 200 lbs of skid plates. If your XJ is stock height and has 225 or 235 tires with no skid plates, you might exceed my gas mileage. However, my XJ does have the advantage of only $40K miles on it. It's mechanically like new, probably better-than-new considering being barely broken-in, well tuned, and having upgrades.

Weather (temp and humidity) affects gas mileage quite a bit. So all tests should be done in similar weather so you can evaluate the effects of each mod. I like to do most of my gas mileage tests during warm dry Summer days when temp is between 75F to 85F (typical Summer days in my climate). I also run Winter tests, but I fully expect less gas mileage during Winter because it's colder. Also be aware that increased humidity reduced gas mileage because your car has to push through more water molecules in the air, which creates more drag. Rain or wet pavement reduces gas mileage because it increases drag.

Many/most mods help in any climate or season, but some are for Summer in hot climates, and some are for Winter in cold climates. I run separate Summer and Winter tests because of these reasons, and because it's a good idea to find out how climate affects gas mileage.
For examples: Hood vent(s) are intended to help during hot dry Summer conditions. A grill cover is intended to help in cold Winter conditions. Since my climate varies from moderate to cold, I do more Winter mods and less Summer mods. My only Summer mod is a large upgraded radiator. A large radiator helps in Summer and doesn't cause any problems in Winter.
See the sections below for Hot Weather Mods and Cold Weather Mods.

Do one mod at a time, reset engine computer, drive it for 2 to 3 weeks, then retest gas mileage (in similar temperature and weather conditions as prior test, on same road, at same speed). Don't do the next mod until after you've gone through that process with the prior mod.

This way you learn how that mod affects performance and gas mileage. If it didn't help, or if it caused a reduction in performance or gas mileage, it's easy to diagnose which mod is the problem because you only did one mod during that test period. Take notes! Write things down or you won't remember later.
If you don't reset the computer, you'll need to drive for at least 10 weeks before testing gas mileage. The computer relearns-adapts to a mod faster if you reset the computer. I'm basing my testing time recommendations on how long it took my XJ's computer to relearn after each of my mods when I reset computer, and when I didn't reset computer.
If you do multiple mods in a test period (more than one mod at a time) you won't know how much improvement came from each mod, and if there's a problem it'll be difficult to diagnose.

Only do one mod per test period!

Instructions for resetting computer adaptive memory can be found later in this thread, and many other threads on this forum and other forums (and yes there is more than one correct way to reset it). However, I'm not sure I trust the goofy rigamaroll procedure recommended all over the Internet for resetting computer. The Internet recommended method might work - I'm not sure. So I go to my local mechanic and he plugs in his laptop and resets my engine computer. It takes him less than 20 seconds and he doesn't charge me for it. However, I must warn you that most mechanic shops charge some ridiculous $50 minimum to reset computer.

Beginning of original OP starts here:
When I bought my XJ, it was getting 15 mpg highway, even though my XJ showed no codes and ran reasonably smooth.

Plugs, cap, rotor, and plug wires needed replacing. Did those things. Those things got me from 14 to 16 mpg highway. A little improvement, but not much. I was disappointed.
Edited in Later: I wish I had used the Kolak tune-up kit aka ignition upgrade kit for Jeep 4L engines. I think it sounds excellent and would be a great set of tune-up parts that would help gas mileage and power for sure, and might even help more than other brands of tune-up parts would. See post 1031 and 1992 for more info about Kolak tune-up/ignition kit and where to buy. Around $230 including shipping (includes upgraded MSD coil, upgraded plug wires, upgraded plugs, and quality cap/rotor with brass contacts).

Edited in Later: 6 months later I bought the Kolak tuneup/ignition kit. It's excellent. Gap plugs to 0.045. My engine starts easier, runs smoother, has more lowend torque and high rpm HP, and it helped my gas mileage a little. The improvements are especially noticeable in cold weather.

That is comparing to the brand new Oreilly tuneup parts I had before, which were of high quality, but no where near the upgraded performance of Kolak kit.
Then Motorvac, new NGK or NTK oxygen sensor, and 205F Stant thermostat. Those 3 changes were done on same day. So I don't know how much each thing helped, but the combination improved my gas mileage several mpg.
Motorvac is a top-end engine cleaning treatment done at mechanic shops where they use a Motorvac machine/pump to pump (under pressure) Motorvac cleaner through top-end of engine to clean out carbon deposits. Works much better than Seafoam.

After Motorvac cleaning, change motor oil (just in case some of cleaner got into bottom-end of engine and contaminated oil).
This is why I waited until my motor oil was due to be changed anyway, then did the Motorvac, then changed motor oil.

Preliminary Preparation
Clean Top End of Engine: I recommend a professionally done Motorvac engine cleaning as the best place to start. It cost me $125. It'll gets the topend of your engine (valves, pistons, etc) clean, which is the best place to start, IMO. There are other brands offering same. I suggest using one that is professonally done because they have the special machines to go along with the chemicals.

Immediately change your motor oil after the Motorvac cleaning to get out any cleaning chemicals that might have gotten into engine. I recommend some excellent motor oil choices below. If you have some other preference for motor oil, then use what you want, but do change the motor oil immediately after the Motorvac topend engine cleaning.
I recommend amy good 0-40 synthetic Motor oil if you live in a moderate climate, or a cold climate. Mobil One 0-40 is what I use. Mobil One, Lucas, Valvoline, and Redline all make excellent 0-40 motor oils and I'd be happy using any of them. Valvoline 0-40 is available in Europe. Those others are available in USA.

I recommend Lucas or Redline 0-40 if you live in a hot climate, or Lucas, Redline, or Mobil One 5-40 if you live in a very hot climate.
Clean Bottom End of Engine: Changing the motor oil after cleaning top end (Motorvac) is to get out any cleaning chemicals from the Motorvac cleaning they have have contaminated your motor oil. Changing to a synthetic motor oil will clean the bottom end of engine (as well as lubricate better).

Synthetic motor oils have more detergent than conventional oils. Synthetic will clean the bottom end of your engine. That is a good thing, but has a risk. Loosening the crud means crud will begin floating around in oil and could damage a main bearing. For that reason, it's necessary to change the synthetic oil and filter after 200 miles (if you were previously using conventional oil) and again after 500 miles. After that you can change synthetic oil and filter every 4000 miles, or you can change synthetic oil every 8000 miles and filter every 4000 miles. The synthetic oils are rated to last more than 12,000 miles as long as filter changed at normal change interval (4000 miles). So changing synthetic oil every 8000 miles and filter every 4000 miles is quite safe and conservative and makes the cost of the synthetic oil affordable.

Synthetic Oils: Using all synthetic lubricants will reduce wear and help gas mileage slightly, and in case of engine will help the bottom end stay cleaner.

When you get to it, change differentials to a good 75-90 synthetic gear oil (I prefer Valvoline 75-90) because this will help lubrication and reduce hydraulic resistance in the differentials. Gas mileage will improve a little just from this. Also change your tcase and transmission to synthetic oil. I'm using Mobil One synthetic transmission oil in my auto transmission. Amsoil also makes synthetic auto tranny oil the XJ can use. My 242 transfer case has the same Mobil One synthetic transmission oil in it. I've notice quieter operation, smoother shifts, and a slight increase in gas mileage. If you have a manual transmission, use 75-90 gear oil in transmission and transfer case (I recommend Valvoline 75-90).

Tuneup: (basic tuneup using quality auto store parts)
A tune up using quality parts (including new plug wires) from any auto store is good and will help (around $80), but the best is a tuneup using Kolak Kit (around $230) tuneup and ignition upgrade parts (includes new upgraded plug wires, coil, spark plugs, and highend brass contact cap and rotor. See above where this is discussed.
For Jeep 4L that have a distributor (early 99 and older XJs): If you tuneup using parts from auto store (instead of Kolak kit) make sure you get a distributor, cap, and rotor that have brass contacts, and use the stock Champion Copper spark plugs gapped to 0.040 for best results because that gap will perform better than stock recommended 0.035 gap.

If you use Kolak Kit, for best results gap plugs to 0.045 if you have a distributor ignition (early 99 and older XJs), or 0.04 if you have a distributorless (coil on plug) ignition (late 99 and older XJs).
For Jeep 4L that have "coil on plug" aka "distributorless": I'm not sure what plug gap is best if you use tuneup parts from local auto store.
Install a new air filter. Preferably an upgraded high flow dry filter (AFE). It's preferable to do this at the time of the tuneup as part of the tuneup. If you can afford it, install a cold air intake that uses a high flow dry filter (AFE).

Upgraded Tuneup and Upgraded Ignition System - Kolak Kit: (use Kolak Kit for a quality basic tuneup PLUS an upgraded ignition system).
The Kolak Ignition Upgrade/Tune Kit provides parts for a tuneup and ignition upgrade in one step. The Kolak Kit improved my cold start, especially on Winter days, torque at all rpm, especially lowend torque, and it improved my gas mileage a little bit too. I'm comparing to how my XJ was when it was already freshly tuned with quality local auto store parts vs Kolak Kit. I highly recommend Kolak Kit for all 4L Jeeps. It's one of the best, most cost effective mods you can do and it improves both performance and gas mileage. It's an effective upgrade in all climates, but especially so in moderate and cold climates where the extra spark helps fully ignite gas, even when when engine is cold.

Kolak Kit (around $230) tuneup and ignition upgrade parts (includes new upgraded Magnacore 8.5mm plug wires, upgraded MSD coil, Autolite Single Platinum spark plugs, and highend cap and rotor with brass contacts. It's excellent. Gapped my plugs to 0.045 (as recommended by Kolak for Jeep 4L with distributor type ignition). My engine starts easier, runs smoother, has more lowend torque and high rpm HP, and it helped my gas mileage a little too. Those improvements are most noticeable in cold weather, but also benefit me in all seasons.
To buy a Kolak Ignition Upgrade/Tune Kit email Nick at: Kolak at

However, don't buy his Kolak exhaust products unless you've read this entire OP, and ideally the entire thread. His exhaust products are excellent quality 2.5" exhaust products which are ideal for strokers, but if you're not going to stroke, you might be better off with a lower cost (but excellent) Dynomax 2.25 exhaust. This also depends on whether you have a stock or aftermarket header, and which one. This is explained a more later in this OP, and a lot more later in the thread. For now, stay with stock exhaust until you know more about that topic.

Upgrade Air Filter (in stock air box)
Install a high flow DRY air filter in your stock air box. I don't trust oiled air filters because I fear their oil might contaminate engine sensors in the intake. My fear may be unfounded, but might be valid. Why risk it? I prefer a high flow dry filter like the one AFE makes. An AFE dry filter for your stock air box is affordable and effective for improving performance and gas mileage a little bit. You can upgrade your stock air box by drilling 3/4" to 1" holes in the bottom of it.

If you can afford (around $300), take it to the next level with an upgrade to a high flow cold air intake (I prefer AFE Cold Air Intake).

Upgrade Fuel Injectors (to modern 4-hole injectors)
I've been reading about guys installing modern 4 hole fuel injectors from Dodge, Ford, Bosch, and others. I want to try some efficient modern 4 hole injectors. See page 14, post 209 for injector(s) part number(s) for your year 4L.

Edited in Later: I installed Dodge/Bosch 789 injectors and love them. Bought them from Motorman on Ebay for $190 including shipping. My engine starts easier, runs smoother, has a little more lowend torque and a little more highend HP performance, and improved my city and highway gas mileage a little bit.
I'm comparing my new 789 injectors to my low mileage stock injectors that had only 35K miles on them and were cleaned recently. 789 injectors outperform new(ish) good condition stock injectors. Your Jeep might need 789 or 703 or other. See page 14, post 209 for injectors part number(s) for your year 4L.

Thermostat (upgrade to the best for your climate)
On an LJ I used to have, it gained 1 to 2 mpg (depending on temp/weather/season) from installing a Stant 205F thermostat (part 13352). That's a good example of a Stant 205F thermostat in a Jeep 4L engine improving gas mileage in a moderate to cold climate.

A 205F thermostat is helpful in moderate climates to cold climates because it allows engine to run at (or near) full 210F op temp year round (instead of under-heating at 185F to 195F). IME, this is a big help in moderate to cold climates and improves gas mileage, especially during Winter. Over the last 20 years, I've done this on 3 Jeeps and 2 cars in a climate that varies from moderate to cold. It also works fine in Summer where we usually have 75F to 85F days, and sometimes mid 90s. Occasionally it gets up to 105F here in Summer. I find a 205F thermostat helps a little on cooler Summer days, more help in Spring and Fall, and a great help in Winter. On our occasional hot Summer days it's no help, but doesn't hurt anything. I'm using a Stant 205F thermostat (part 13352), which is same as Napa 205F and Gates 205F (Stant makes those brands).

If you live in a frequently hot climate your engine is already at, near, or above full op temp (210F) with stock 195F thermostat. If you live in a hot climate, a 205F thermostat offers no benefit and would probably be a detriment. If you're in a hot climate, I recommend upgrading to Stant 195F Superstat (part 45359), which is same as Napa 195F Superstat (Stant makes Napa). It opens at stated 195F temp and maintains a more stable, consistent temp than most thermostats. A stable, consistent temp helps the engine's computer fine tune combustion for better gas mileage. Another great thermostat is Motorad 195F Highflow (part 2000-195).

If your thermostat is to hot for your climate, your fans will work harder, which increases drag on engine and reduces gas mileage. It takes more power to turn mechanical fan when fan clutch engaged, which increases drag on engine. It takes electricity to turn the electric fan, which makes alternator work harder, which increases drag on engine. Increased drag on engine reduces gas mileage and performance.
XJs in very hot climates will have their fans turning a lot no matter what thermostat they use. For that situation l recommend a Stant 195F Superstat or Motorad 195F Highflow thermostat (high performance thermostats) and passive cooling mods: such as a larger radiator, oil cooler, transmission cooler, and/or hoodvent(s). Passive cooling mods don't use energy and therefore don't decrease gas mileage. In fact, in a hot climate, passive cooling mods can increase gas mileage by reducing the amount of work the engine fans do.

Gas burns more efficiently at hotter temperatures, but you want engine cool enough so your fans aren't coming on. The ideal situation is engine running as hot as it can without engaging mechanical fan clutch or turning on electric fan. The fans engage at around 214F to 216F. Gas burns really efficiently at engine temp of 208F+, and especially 210F+ (IME). The ideal engine temp is anywhere between 208F and 213F (IME). That's hot enough for an efficient fuel burn, but cool enough so fans don't come on.

Upgrade to High Flow Air Intake
I initially installed a high flow upgrade filter in stock air box which gave slightly improved performance and +0.9 mpg. About 4 months later I installed an AFE "Cold" Air Intake (high flow) and gave additional performance and an additional +0.7 mpg. So that's almost 1.6 mpg better than stock filter in stock box, and almost 1 mpg better than upgraded filter in stock box. It helped performance more than gas mileage, but it helped gas mileage some.

I installed an AFE "Cold" Air Intake (actually slightly cooler air at higher flow) and I have noticed slightly more power and highway gas mileage. It's a great high flow air intake, but not a true CAI. It's warm air drawn from the same place as the stock air intake, which is perfect for gas mileage. High flow warm air is ideal for gas mileage in a moderate to cold climate, or any climate with seasons. I bought the dry filter for it because I don't like oiled air filters (I fear oiled filters might dirty sensors). See post 150 for more info on types of air intakes.

Insulating Air Intake
A few months later, I decided to take it to the next level with an idea I got from Dino Savva's Jeep website mod diary. I installed additional heat insulation on the exterior of my AFE cold air intake's heat shield and air tube. I did this using stick-on heat shields on the AFE metal heat shield and header wrap on the AFE intake tube, and heat reflective tape on the heat shield seams (it took more than one stick-on heat shield to cover the AFE metal heat shield because my auto store didn't sell big enough ones).

The extra heat shielding slightly improved my lowend torque and city gas mileage. It made no difference to my highway performance or my highway gas mileage. It helps at speeds below 20 mph (when heat starts building up under hood on Summer days - like at a stop light, or when crawling). It was an effective cheap mod.

If you can't afford a cold air intake and still have the stock air box, put a high flow filter in your stock air box (I recommend AFE high flow dry filter) and add heat insulation to the stock air box (top-lid, sides, and air tube; but do NOT insulate the stock air box bottom where you should have already drilled 3/4" to 1" air holes to increase flow). You can use stick on heat shields on stock air box, header wrap on stock air tube, and heat reflective tape on shield(s) seams. I recommend teflon header wrap.
Insulating intake is definitely helpful in hot climates, doesn't make much difference in moderate climates, and might be a hindrance in cold climates (where engine underheating is likely). So consider your climate before deciding whether to insulate intake. has a good selection of stick on heat shields and teflon header wrap. Most local auto stores have a poor selection. Buy more header wrap than you think you'll need. It always requires more than you thought. I think around 15 to 20 ft is enough for intake tube. So buy 20+ ft to be sure. It's better to have to much than run out before finished.

Edited in Later: I rewrote the exhaust section to replace my earlier theories with real world results after trying and testing various exhaust setups.

The Dynomax brand cat back exhaust is good breathing, easy to install, quality product, low cost, quiet, and reasonably rust resistant. I tried both the 2.25" and 2.5" pipe diameter versions to see which works best. With stock header and stock cat (low budget approach) a 2.25" cat back works best for lowend torque, general performance, and gas mileage. With an aftermarket Try-Y header (such as Thorley HDX-634Y) (a higher budget, higher performance solution) the 2.5" cat back (with Magnaflow 2.5 in/out high flow cat) works best. When I say "best" I mean of the options tried by myself and another person in this thread (Christianbe). Between the two of us we tried several exhaust combinations. I tried 2 on my earlier Jeep, 3 on my current Jeep, and he tried 2 or 3 on his current Jeep.
Edited in Later (after using both diameters of CB with stock header on my XJ): The 2.25 cat back improved my seat of pants performance at mid and high rpm compared to stock exhaust, and lost nothing at low rpm compared to stock exhaust. The 2.25 cat back improved my highway gas mileage 1.5 mpg, which is pretty good considering it cost only $150 incl shipping and substantially improved my performance. I can much more easily drive up mountain passes nice and fast now and easily pass cars doing so.
I didn't test city gas mileage, but based on seat of pants feel taking off from a stop, I think stock exhaust is best for city mpg (least gas pedal pressure to get moving), and 2.25 cat back almost as good. The 2.5 cat back took a lot more gas pedal to get moving from a stop - so I assume the 2.5 cat back got the worst city gas mileage.
For a low cost, effective improvement to performance and gas mileage, I recommend Dynomax 2.25 cat back system (with stock header and stock cat, assuming they are functioning properly - no leaks or clogs). The Dynomax 2.25 cat back system (part# 17340, costs around $150 including shipping) from or, or anywhere you get a good price. it's a great price and easy install for 93 to 01 XJ.
For somewhat higher cost with even greater improvements to performance and an untested amount of gas mileage improvement (I'd bet it improves gas mileage), I recommend Thorley Header (part# HDX-634Y, around $515 from Kolak or others with good price), Magnaflow High Flow 2.5" in/out Universal Cat (part# 99006HM, around $80 from Kolak, or others with good price), and Dynomax 2.5" cat back system (part# 17463, $107 from or others with good price). Prices include shipping.
Note for CA and NY: Magnaflow High Flow Cat 2.5" in/out (Magnaflow calls it Universal Cat) part 99006HM is a "Federal" cat for use in 48 states, but not in CA and maybe not NY. The high flow cat for CA has a different part number (doesn't flow quite as well, but flows pretty good). NY might have a different Universal cat part# too, not sure about NY. You can look up CA and possibly NY part# yourself at Magnaflow's website by model and year of your Jeep.
Don't worry about which model years of XJ that Dynomax' website says fit each of their XJ cat back systems. Either system will fit any year XJ. Dynomax cat back system 17340 is a 2.25" cat back. Dynomax cat back system 17340 is a 2.25" cat back. The 2.25 CB is excellent with stock header and cat. The 2.5 CB is even more excellent when combined with Thorley header and Magnaflow High Flow cat. Do NOT combine the 2.5 CB with stock header (that combination gives weak performance and does not help gas mileage. I installed and tested it.).

I'm not theorizing. I installed and tested both 2.25 CB and 2.5 CB with stock header (and a few other exhaust setups/combinations). Christianbe (participant in this thread) is using Dynomax 2.5 CB with Thorley header and Magnaflow High Flow cat. So the exhaust advice here isn't theory or guessing. We've installed and tested these things IRL.

The only interesting exhaust setup combination we didn't test is Thorley header with Magnaflow 2.5 in/out high flow cat and 2.25" Dynomax cat back. I plan to install and test this combination in future.

For the very best and most rust resistant 2.5" cat back exhaust system I'd recommend the Kolak cat back exhaust or a Thorley 2.5" cat back exhaust. Either is the caviar of cat backs, IMO. Both are high grade stainless. The Kolak exhaust pipe is made by Mike Leach (of Leach header fame). Those are (IMO) the best 2.5" cat backs. They cost several times more than a 2.5" Dynomax does. The performance of the Kolak/Leach and Thorley cat backs is same or very similar as Dynomax. The advantage of the Kolak/Leach and Thorley cat backs is they are very rust resistant and would probably last longer than your Jeep. The Dynomax is aluminized steel and is rust resistant, but not in the same league of rust resistance. The Dynomax is excellent if you live where roads are NOT salted. Even if you live where roads are salted, Dynomax would probably last 5+ years and only costs $107 incl shipping.

For the best 2.25" cat back, the Dynomax is probably as good as it gets, and that's pretty darn good. It's only $150 incl shipping.

For more cat and cat back exhaust info see post 2511.

Downpipe aka Crossover Pipe
Exhaust discussion should include the downpipe. This is the pipe that connects the header to the cat.

The stock downpipe (crossover pipe) from header/manifold to cat has a huge dent that restricts exhaust. I plan to replace with a dent-free downpipe. This cheap mod will help power and possibly gas mileage when a high flow cat back exhaust is installed.

Edited in Later: For $80 installed my local exhaust shop made me a 2.5" non-mandrel bent dent-free downipe made of mild steel that works great (after a few fitment/clearance adjustments explained in following paragraph). It would be even nicer if it was mandrel bent and made of stainless steel, but what I got works great and only cost $80.
Warning: Make sure to tell your exhaust shop that stock routing for dent free downpipe might not clear the driver side UCA or diff output yoke during suspension uptravel or articulation if you have no bump-stop extension. Figure out a new/better routing that will clear UCA and need locate oxygen sensor where it's not above front differential yoke (so it doesn't get hit during suspension uptravel). I learned the hard way, but some free simple adjustments saved the day for me, and it's now working great for me, even though I have stock routing and no bump-stop extension.
Edited in Later: Although my dent-free downpipe follows stock routing, and I have 1.25" lift with no bumpstop extension, I have no clearance issues with UCA or diff output yoke (after my local exhaust shop made two minor adjustments).

Insulating Exhaust
This is an interesting topic worth looking at. It improves performance by improving exhaust flow and scavenging. I suspect it might improve gas mileage too. I plan to test this on my XJ sometime in future.

I plan to use exhaust-header wrap, which is the low cost way to do this. They don't salt roads where I live. So exhaust/header wrap will be fine for my XJ. Don't use wrap if you live where they salt the roads because it will retain salt and accelerate rusting.

The higher $ approach would be to send in your downpipe (crossover pipe), and perhaps your tail pipe as well, to a ceramic coating service that will paint/coat it for you. That's your only sensible option if you live where the roads are salted in Winter.

I bought a Thorley ceramic insulated header, which I haven't installed yet. After it's installed my header will be insulated. Then I'll use header wrap on my downpipe (crossover pipe) and wrap all the way to the cat. I probably won't wrap the tail pipe, but I might. I need to do some more research to decide if I'd want to wrap the tail pipe too. I'm not sure if wrapping cat and muffler is a good or bad idea. I'll have to ask my loca exhaust shop for advice. Luckily they are a performance exhaust shop and familiar with insulating exhaust. See post 2543 for my limited knowledge on this topic. I plan to learn more in future.
Note: Insulating header and down-pipe aka crossover-pipe is also a passive cooling mod because it reduces underhood temps. Reducing underhood temp is helpful in hot climates, doesn't make much difference in moderate climates, and is a hindrance in cold climates (where engine underheating is likely). So consider your climate before deciding whether to insulate header and downpipe.

Hot Weather Mods (helpful for hot dry climates, and when crawling off road during Summer in any climate)
Passive Cooling Mod(s)/Upgrades (primarily for hot climates, but some passive cooling mods work well in all climates. A larger radiator is a passive cooling mod that works well all climates.)
Passive cooling mods use no energy and they help cool the engine. So then fans run less often, which reduces drag on engine, which increases gas mileage and power. Examples of passive cooling mods are: A larger radiator, engine oil cooler, transmission cooler, hoodvent(s), and insulated exhaust (insulated header and/or downpipe aka crossover pipe). Passive cooling mods are helpful and efficient if you live in a hot climate.
However, in a moderate climate passive cooling mods aren't much help, if any. In a cold climate certain passive cooling mods should be avoided because they will cause underheating. Underheating reduces gas mileage substantially.

A larger radiator is my favorite passive cooling upgrade because it works well in all climates and seasons. My radiator is upgraded. Likewise with transmission coolers and oil coolers IF they are thermostatically controlled so they don't cause underheating during cold weather. My XJ is a tow package XJ an upgraded radiator stock and it might also have a transmission cooler, but I'm not sure.

The ideal type of hood vents are vents you can open in Summer and close (or plug) in Winter. That way they help in Summer without hindering in Winter.
Headers and/or downpipe (crossover pipe) can be insulated with ceramic coating (best) or with header wrap (low cost). Header wrap is fine if you live in an area where they don't salt the roads.

I think an insulated header is very helpful in hot climates, doesn't matter much in moderate climates, and is a hindrance in cold climates. If your engine runs to hot or cold, gas mileage suffers.

Hoodvent(s) (for hot dry climates only, unless you have a good way to close/plug the hoodvent during cold or wet weather)
A great mod for Jeeps in hot dry climates is a hood vent located on the front third of the hood. Ideally approx right above the front of engine. You don't want a hoodvent near the windshield because high pressure air from windshield wouldn't allow enough much air to exit the hoodvent. The beauty of vents is they cool the engine compartment without using any power to do so. It's free cooling, which will save gas because your fans won't need to work as hard. The ideal type of hood vents can be opened in Summer and closed during Winter (and closed on rainy days).
Warning about hoodvent(s) in cold and/or wet climates: I had a hoodvent installed on my LJ. It was great during crawling off road in Summer, but in Fall, Winter, and Spring it drained rain from the hood onto my fan and alternator pulleys, which rusted them causing lots of squealing. It drove me nuts. The hoodvent also caused underheating during Fall, Spring, and severe underheating during Winter. Underheating greatly reduces gas mileage. I planned to make a removable seal/plug for the hoodvent during Fall, Winter, and Spring; but I sold that LJ before I implemented a solution. So now that hoodvented LJ is someone else's problem. If you live in a cold or wet climate, don't install a hoodvent unless you have a plan/way to seal it shut during cold or rainy weather. I didn't need a hoodvent in my moderate climate. I never should have installed a hoodvent on my LJ. I learned my lesson. My XJ will NOT get a hoodvent.
The ideal engine temp keep engine as hot as possible without triggering the fans. 208F to 213F is ideal for this since the fans trigger around 215F. I prefer around 210F, and am happy anywhere from 208F to 213F. To accomplish this, XJs in moderate to cool climates need a Stant 205F thermostat (and a grill cover for very cold climates), while XJs in hot climates (regularly 96F+) need a high performance 195F thermostat, a larger radiator, possibly an oil cooler and/or transmission cooler, and if it's a hot dry climate a hood-vent(s).
My XJ's 4L engine runs 208F to 210F most of the year (205F on coldest Winter days with heater on). When weather occasionally gets above 95F here, my 205F thermostat is fine for protecting my engine from overheating because my stock fans seldom let engine get above 215F and never above 220F (engine damage doesn't occur until above 230F). However, my gas mileage suffers when fans run a lot on those occasional 95F+ days, but in my area we only get 0 to 10 days a year that are 95F or hotter.

Cold Weather Mods (for Winter in moderate climates; and Fall, Spring, and Winter in cold climates)
Cold weather gas mileage will never be as good as warm weather gas mileage, but with the cold weather mods described in this section, Winter mpg (and performance) will be as high as possible.

205F thermostat
A 205F thermostat helps gas mileage during moderate to cold weather. It won't hurt anything in Summer. If you're in a moderate or cool climate this is an excellent cheap mod. I recommend Stant 205F thermostat. If you're in a hot climate, skip this mod. See full explanations in the "Thermostat" above.

Modern 4 Hole Fuel Injectors
Modern 4 hole fuel injectors improved my torque, power, and fuel economy in all seasons, and is especially helpful in cold weather. Injectors with 4 holes provide a finer fuel mist (compared to stock 1 hole injectors). The finer mist ignites more easily and efficiently, especially in cold weather. See full explanation in the "Upgrade Fuel Injectors" section above.

Kolak Kit
The Kolak Kit Tune-Up and Ignition Upgrade improved my torque, power, and fuel economy in all seasons, and is especially helpful in cold weather. The stronger, larger spark ignites fuel more easily and efficiently, especially in cold weather. See full explanation in "Upgraded Tuneup and Upgraded Ignition System - Kolak Kit" section above.

Stop the Defroster from Triggering Engine's Electric Fan
The defroster (or defroster and heater combination) triggers AC compressor to cycle on whenever the AC pressure drops below op pressure. Why is AC compressor needed at all for defroster? It theoretically helps dry (dehumidify) the defroster air to faster defog the windshield. I've been told it only does this when defroster is on and outside air temp is above 40F. I'm sure if that's correct. I'd like to leave this this part of system working for better/faster defroster performance and because keeping gas/fluid circulated in AC keeps the seals good longer.

However, every time the AC compressor cycles on, the engine's electric fan cycles on. This is inefficient for 3 reasons: AC compressor puts drag on engine when it cycles on, it requires power to run the electric fan, which increases alternator drag on engine, and in cold weather city driving the fan makes engine take much longer to warmup, and it prevents engine from warming up as much as it otherwise would, especially when driving a lower speeds (city or rural roads) in Winter. This wastes gas in multiple ways. More drag on engine and an underheating engine are both fuel wasters. So this situation is quite a significant fuel waster, IMO.

What I wanted to do was leave the AC compressor functioning normally when using defroster (as described above), but prevent it from turning on the engine efan. However, I haven't yet figured out how to do that.

What I have figured out is this: My AC system is shot. It's out of AC gas and has one or more leaky seals. So I unplugged the electric wire that goes to the AC compressor. This prevent the compressor from coming on and prevents it from turning on the efan. Efan still works when/if needed to cool engine. This mod (unplugging wire from AC compressor) significantly improved my Fall, Winter, and Spring gas mileage so it's closer to what my Summer mpg is.

I have also discovered that my defroster can still defog the windshield just fine. I don't know if it defogs as quickly as before, but it defogs plenty fast enough. So no complaints there. Also, I don't need AC in my moderate to cold climate (my AC was already broken). I'm quite happy with this mod.

Grill Cover
I want to make or buy an adjustable grill cover for the colder months. Like the coldest 4 months of the year. Adjustable grill covers help gas mileage 2 ways. A grill cover helps engine reach full op temp quickly during cold weather, and improves aerodynamics by keeping unnecessary air from entering engine compartment, thus reducing drag.

Edited in Later: My Winter climate isn't cold enough to ever need to completely cover my grill. So a full grill cover is unnecessary and unwanted in my slightly cold Winters. My engine would overheat with a full grill cover. However, with no grill cover at all, my engine underheats during the coldest Winter days. I'm going to make a partial grill cover that covers the middle 3 openings in my XJ grill. That is what many people in moderate climates have done. I'm going to follow in their footsteps.
A full grill cover is only for artic Winter conditions, which I don't have.
Edited in Later: I figured out a plan to make an aerodynamic grill cover. Will start work on that soon. It will cover middle 3 grill openings, and it's only adjustment will be removal when weather warms up. 10-6-2014

Chips, Tuners, Programmers
Chips and store bought tuners have a bad rep every forum because they don't work or sometimes even cause problems. This is covered in a discussion somewhere later in this thread. If I have time, I'll add the post #'s here where that is discussed by several JF members.
However, custom tuner FlyinRyan has a great reputation at Jeep forums for getting great results improving performance on stock and modified 4L, and strokers. Most people go to him for performance boosts and he delivers, often while maintaining or slightly improving gas mileage. I wonder what he could do for gas mileage if that was the primary goal? See post 2512 for more info.

For anyone who wants to use FlyanRyan's services, I suggest waiting until after all your bolt on mods are done (for a 4L), or after all your physical mods are done (for a stroker). Then he can tune to get the most out of your other mods.

Important: After experimenting on two of my Jeeps with several different exhaust systems and talking at length with FlyinRyan, I give the following exhaust advice relating to stock tune vs Ryan's custom tuning:
If you are NOT going to use FlyanRyan's turning services (leaving stock tune), for a modified 4L I recommend keeping stock header (and head) and stock cat [or use stock equivalent replacement cat - I like Magnaflow (stainless) for stock equivalent replacement cat (2.5" in/2.25" out)] and using Dynomax 2.25 cat back (aluminized).

If you are going to use FlyanRyan's turning services, for a modified 4L I suggest using Thorley HDX-634Y header (stock head is fine), Magnaflow (universal - stainless) high flow (2.5" in/2.5" out) cat, and Dynomax 2.5 cat back (aluminized), or if you want an all stainless exhaust then get Kolak exhaust (same Magnaflow stainless high flow cat and a very highend stainless 2.5" cat back).

Regardless of whether you use FlyanRyan's turning services or not, for a 4.5 to 4.7 stroker I suggest using Thorley HDX-634Y header (stock head OK, but ported head is best), Magnaflow (universal - stainless) high flow (2.5" in/2.5" out) cat, and Dynomax 2.5 cat back (aluminized), or if you want an all stainless exhaust then get Kolak exhaust (same Magnaflow stainless high flow cat and a very highend stainless 2.5" cat back).
Dynomax 2.5" cat back is very cheap ($107) and good. The Kolak cat back costs more, but is a good price for what it is, and it's the best there is, IMO. The Kolak cat back is part of the Kolak exhaust system and includes Magnaflow high flow (2.5/2.5) cat.
Edited in Later: I've never taken the time or spent the money to use FlyanRyan's custom tuner services, but I'm confident that if I did, my already excellent performance would get even better, and my already excellent gas mileage would get slightly better. He's the real deal for sure. I just haven't followed up on this because of time and money constraints, and because my XJ is already doing so awesome that I don't feel the need for more improvement, but I'm sure Ryan could get me significantly more if I want it.

Edited in Later: I still believe in FlyinRyan, but all his tunes are for premium gas only. I don't want to use premium. So I won't be using FlyinRyan's services.
The mods I've already done work OK on regular unleaded, and excellent on midgrade gas. Midgrade is 30 cents a gallon cheaper than premium in my area. So I'm staying with the stock tune and my bolt on mods because that combination is working great and I can use relatively low cost midgrade gas. I could even use regular gas if I wanted to, but my 4L Jeep runs best on midgrade.

62 mm Throttle Body
I don't know if installing a 62 mm TB would affect gas mileage. I've never tried this mod. There are many user posts/reviews on the Internet forums claiming it helps, many claiming it made no difference, and a few claiming it reduced lowend torque and gas mileage. I have no idea what effect this mod would have on gas mileage. I suspect it'd reduce city gas mileage because the gas pedal would be touchier. I can't even guess how it might affect highway gas mileage. I'm considering trying this mod as an experiment to see how it would affect my gas mileage. For those who just want results and don't like to experiment, skip this mod for now since we have no idea what effect it would have. I plan to (eventually) install and test a 62 mm TB and then post my results.

Note: I'm now getting 25 mpg from the mods I've already done. I have not yet modified the throttle body. i.e. - I'm getting 25 mpg with stock throttle body.

Aftermarket Header
I don't know yet if there's any benefit to installing an aftermarket header upgrade if your stock header is still good, but I'd like to know. I think Thorley makes the best headers. I plan to (eventually) install and test a Thorley header and then post my results.

Note: I'm now getting 25 mpg from the mods I've already done. I have not yet modified the header. i.e. - I'm getting 25 mpg with stock header.

Replacing Mechanical Fan with Electric Fan
Replacing mechanical fan with electric fan is an efficiency improvement in a moderate to cold climates (because electric fan seldom comes on in cool climate), but offers no efficiency benefit in hot climate (where fans run often).
Edited in Later: The downside of converting mechanical fan to electric is expensive to hire it done, and complicated to do yourself. The biggest complications are getting it to fit, wiring it properly, and figuring out an appropriate and reliable temperature sensor to automatically and reliably trigger it. For these reasons, I might not do this mod, and I recommend caution to others considering this mod. A lot of people have had difficulty getting this mod done and ongoing difficulty getting it to function properly. Many (perhaps most) people never get it working properly. I know a local radiator shop that will do this mod for me for $750, incl parts and labor, and it'd probably be reliable; but it's not worth the expense (or risk) to me at this time.

So although replacing mechanical fan with electric fan is good in theory, in actual practice it's NOT easy or affordable to get working properly. It's a risk. If you do this mod, it should be the LAST mod you do because there are many simpler, easier, lower cost mods that will improve gas mileage.
Aerodynamics This is covered repeatedly later in thread, but I'll touch on a couple highlights here.
Skid Plates
The smoother the underside of the Jeep the better the gas mileage will be (less wind drag). Some Economodders (at other forums) have covered underside with corrugated plastic (like used to make signs) and have reported good success. I have found that underbody skid plates have been helpful to me in a similar (though less effective) way. Specifically a stock front skid plate that protects tierod, and also my Skid Row Engine transmission skid plate. So although the weight of skid plates reduces city gas mileage a little bit, the smoother underside increases highway gas mileage a little bit. So over all, average gas mileage is unchanged. So don't be afraid to add skid plates if you need off road protection. Also, I have noticed my 200 lbs off full skid plates improve my handling and cornering be lowering my center of gravity.
Lift and Rake
The lower to the ground a car is, the less wind drag it will have. So lower equals better highway gas mileage. However, we drive Jeeps, and many of us, myself included, like to have a lift on it.

Only lift as high as necessary to fit your tires, and no more. A positive rake is when the rear is higher than the front. It has been conclusively proven by tests done by car manufacturers and also a 4x4 magazine tested this by testing the effect of leveling kits on a Grand Cherokee ZJ's gas mileage. 1" Positive rake gave 2 mpg more on highway than a 0" rake did. (0" rake means it sits level). A negative rake would be the worst (front higher than rear).

So when you lift, it will help your highway gas mileage significantly to have a positive rake. A 1" positive rake is ideal, but looks weird on a Jeep. So I setup my XJ's lift to have 0.5" positive rake and that looks good. I think it's giving me around a 1 mpg improvement on highway, though I didn't test my mpg specifically for that.

So try to have a 0.5" positive rake to your lift (if you have lift), but for sure avoid negative rake.
Roof rack, winch, bull bar or brush guard, running boards, mud flaps, off road lights, and spare tire on roof are common items that increase wind drag on highway. Bull bar, brush guard, winch, and spare tire on roof are the worst offenders and any one of those things can reduce highway gas mileage by 2 mpg, IME. I gained 2 mpg just from removing my winch. So remove the things you don't need. I also removed my roof rack and running boards and I have refrained from adding off road lights. A spare tire on the roof is a terrible idea for gas mileage (and center of gravity). If your tire is to large to fit inside your XJ, then mount it on the rear.

Mods to Avoid (if you care about gas mileage)
Avoid active cooling mods (high output fan(s), high volume water pump), high output alternator, and high volume oil pumps because they aren't needed and increase drag on engine, which reduces performance and gas mileage. Avoid any mod that increases engine drag, vehicle weight, or wind resistance; unless you really have a strong need for the functionality that mod provides. If you really need a gas guzzling mod, then you might have to install it and sacrifice some gas mileage.
You don't need active cooling mods because you can do passive cooling mods instead. Active cooling mods hurt gas mileage by increasing drag on engine. A more powerful (than stock) mechanical fan (more blades or clutch than engages at lower temp) increases drag on engine. A more powerful electric fan uses more electricity, which increases load on alternator, which increases drag on engine. Likewise with adding extra fans. Passive cooling mods help gas mileage (if you're in a hot enough climate to need cooling mods). Passive cooling mods use no power and don't create any drag on engine. In fact, passive cooling mods reduce drag on engine because fans won't need to come on as often.

You don't need a high volume oil pump. The stock oil pump is capable and appropriate.

You don't need a high output alternator for a winch. Buy a more efficient winch (with external disk brake) that uses less a lot less amps (such as Superwinch - low amps, but fast). You don't need a bunch of offroad lights, but if you really must have them, are there more efficient LED lights available? The only valid reason to have a high output alternator is for welding on the trail, IMO. If you aren't welding on the trail, you don't need a high output alternator.

Earlier, I had a winchplate and winch, but it reduced my city and highway gas mileage (costing me -2 mpg highway and -1 mpg city) and I don't really need a winch. So I uninstalled the winchplate and winch and my gas mileage immediately improved to what it was before the winch.

Don't store spare tire on roof (wind drag). Don't install grill guard or aftermarket roof rack (weight and wind drag) unless you really need them and are willing to sacrifice gas mileage. I removed my roof rack, running boards, and winch; which significantly reduced weight and wind drag.

Obviously larger tires and skidplates would be expected to reduce gas mileage, but I installed those mods anyway because I wanted the off road capability. In spite of my 30" tires and 200 lbs skidplates, I get 25 mpg at 60 mph (after various gas mileage improvement mods). I'd get better city gas mileage if I had stock tires and no skidplates, but (thanks to various efficiency enhancing mods described earlier in this thread) my city gas mileage is adequate and my highway gas mileage is excellent.
My 30 x 9.5 tires aren't much taller than stock and barely any wider. Staying with narrowish tires helps. Wide tires are much worse for gas mileage. My 30s aren't hurting my highway gas mileage much, if any. I'm sure they reduce my city gas mileage a little (I estimate -1 mpg city). I've proven it's possible to get excellent highway gas mileage (and good city gas mileage) with 30 x 9.5 R15 ATs (I'm using Cooper AT3, which has a reasonably low rolling resistance), but I think if you go taller (like 245/75R16 aka 31 x 9.5) your gas mileage will suffer some (I estimate -1.5 to -2 mpg highway). If you go wider, your gas mileage will suffer some more. If you go taller and wider, your gas mileage will suffer A LOT. I think 31 x 10.5 R15 would not be ideal for gas mileage. However, the gas mileage mods in this thread would help it be as good as it can be. I think it's possible to get 22 mpg at 60 mph with a 31 x 10.5 R15 AT).

My 200 lbs of underbody skidplates didn't change my highway gas mileage. They added weight but improved underbody aerodynamics and my highway mpg didn't change. I'm sure the weight reduces city gas mileage a little (I estimate -1.5 mpg city), but it's worth it to me to have skidplates. IME underbody skidplates don't hurt highway gas mileage if you don't install anything crazy heavy. A 1/4" thick belly skid is crazy heavy. My skids are all 3/16" and I've got everything covered. If you install skidplates, I highly recommend an XJ front skid to protect tierod and because it improves underbody aerodynamics. A combination engine-tranny skid (like my Skidrow) also improves underbody aerodynamics. Avoid differential helmets/cage guards on front diff because they create wind drag. I'm using a thick (1/4") strong Alloy USA front diff cover, which is same shape and size as stock cover, so it doesn't cause wind resistance and won't hit your trackbar during suspension compression like a big aftermarket cover can.


Does anyone have other gas mileage suggestions? If so, please post them.


Edited in Later: I am using all synthetic oils in my XJ. This is mostly to protect engine and auto tranny from heat and wear, but also to increase gas mileage. All synthetic oils in all areas increases gas mileage an estimated 1 mpg, and reduces wear substantially.

0-40 motor oil is excellent/ideal in 4L Jeep engines for use year round in all climates.
Mobil One 0-40 is what I'm using (although I'd be just as happy with Lucas or Valvoline 0-40).
Mobil One, Lucas, and Valvoline 0-40 are excellent. Valvoline 0-40 is widely used in Europe, but very difficult to find in USA.
Synthetic auto transmission oil in my auto transmission, and 242 transfer case.
Mobil One and Valvoline synthetic auto transmission fluids are my preference.
231 transfer case uses 75-90 or 90 gear oil. I prefer Mobil One or Valvoline synthetic gear oils.

Mobil One and Valvoline synthetic gear oils are great for differentials and for a 231 transfer case, BUT do NOT use these gear oils in a manual transmission because their limited slip additives makes them to slippery for syncromesh shifting to work properly.

Do NOT use gear oil with limited slip additives in a manual transmission. In a manual transmission, only use gear oil that does NOT contain limited slip additive.

75-90 synthetic gear oil front diff (I recommend Mobil One or Valvoline)

75-90 or 75-140 synthetic gear oil rear diff (choose viscosity according to climate and severity of use (towing requires 75-140 or 80-140)) (I recommend Mobil One or Valvoline)

I like Valvoline best in 75-90, and Mobil One best in 75-140 based looking at their spec sheets and their viscosities cold vs hot, and their flash points.

Edited in Later: I have perfect alignment and perfectly balanced tires, which helps gas mileage.

My tire pressure is on the low end of acceptable to soften ride for my bad back. This is NOT helping my gas mileage. I am going to install softer springs to soften ride. Then I will start running more tire pressure, which will gain me at least another 1 or 2 mpg and put me at, or very near, 25 mpg.

I've already increased to 23 mpg from fine tuning and a few of the mods mentioned above. I still have many more mods to do. l'm sure I will get substantially more than 25 mpg when I am finished.

Edited in Later: My XJ has auto tranny, 3.55 diff gears, and 30x9.5R15 AT tires. Later in thread a member with same setup regeared to 3.73 diff gears and his gas mileage improved.

Edited in Later: Later in thread it was pointed out that my auto tranny puts me at some disadvantage for gas mileage. The manual tranny XJ have a built in advanatage for gas mileage.

I have reached 25 mpg with my auto tranny, but if I had a manual tranny I could do even better. Manual tranny 4L XJs are known to get 24 to 25 mpg highway when completely stock. So with economods a manual tranny XJ could potentially get 27 or 28+ mpg at 60 mph.

Edited in Later: I got 24.89 mpg at 60 mph on level highway in test today (Sept 13, 2014). That's pretty much 25 mpg. I still have a few mods to do. I'm hopeful I might get it above 25 mpg in future. See posts 2476 through 2479.

Edited in Later: I got 22.7 mpg at 65 mph on level highway today in gas mileage test (Sept 19, 2014). See post 2515.

The latest gas mileage figures (above) are from completion of most of the low cost mods that are possible. I think that's about the limit of what's economically feasible. I might be able to squeeze another 1 mpg out of it on a low budget. After that, any additional improvement will get expensive and require a higher mod budget.

I could get even higher gas mileage, but it would require some expensive mods to do so. I think around $1200 of additional mods might gain another 2 mpg. I think $3200 of additional mods might gain another 3 mpg. That'd be neat as a hobby, but not cost effective as an investment to save gas money.

I've already taken it almost as far as makes sense economically, but it would be fun to take it as far as I can afford and I probably will.

Edited in Later: My gas mileage and performance are with 89 octane E10 gas, which is the lowest ethanol content available in my area. I don't know if 87 or 89 octane is better since I didn't test that much. I only know what I've been getting with 89 octane.

Edited in Later: Recently I started using 87 octane E10 in Winter, and plan to use 89 octane in Summer. I theorize this is best for my climate, but haven't tested 87 octane enough yet to know. Most of my experience with with 89 octane, which is what I used for all the tests described in this thread. Regardless of octane, I use E10 gas because that's what's available in my area. I theorize that my gas mileage would be better on pure gasoline, but that's not available to me.

Edited in Later: I live in Pacific Northwest (WA State and OR) where gas costs approx $1 more per gallon than it does in most of the rest of the USA. So the $750 of parts money spent on gas mileage mods was a good investment for me since I'll recover that money in about 1 year of driving. My gas mileage has improved from 15 mpg to 25 mpg at 60 mph, or 23 mpg at 65 mph.

Warning: I often edit my posts a few times to get them complete, or to correct errors.

Last edited by Charley3; 02-08-2015 at 05:08 PM.. Reason: Updates after installing and testing more items
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Unread 07-21-2013, 02:39 PM   #2
rockin lil red wagon
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1998 XJ Cherokee 
Join Date: Jun 2008
Location: Hatboro PA
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yea i have heard about people using the 4 hole injectors from a neon to replace the stock 1 hole injector,don't know how much benefit there is, i guess there should be some as the spray will be finer and burn better than a drip.
ah,beer the cause of and cure to all of lifes problems ...Homer J Simpson... I am the guy in the red jeep!
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Unread 07-21-2013, 03:21 PM   #3
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You've piqued my interest. I'll follow this post with anticipation. Good luck!
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Unread 07-21-2013, 03:38 PM   #4
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1997 XJ Cherokee 
Join Date: Mar 2011
Location: Lovettsville, VA
Posts: 192
you should certainly look into a bored throttle body, not a straight through muffler but one with a little more free flow, ditch anything that sticks out on top of your roof (roof racks), certainly run dual elec. fans, a new coil (accel makes a great one) and definitely do the injector upgrade.
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Unread 07-21-2013, 03:42 PM   #5
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Instead of CAI you might look at hot air. This is the way the hypermilers get the computer to lean out the fuel mix even more. I dont find the 4 hole injectors are worth anything over and above quality precision single hole. Look at the flow balance between them instead and get it as close as possible. mine are within 0.3 percent of each other. Wrap your exhaust front to rear. Swap cams to a tamer grind (under .375 lift and around 180 duration at 0.050). Couple that with a 50-52mm throttle body. Swap to the late-model head with raised exhaust ports. Start closing off and smoothing the front end. Reduce the grill opening as much as possible. Belly pan to direct air helps. Run as light as possible.
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Unread 07-21-2013, 03:53 PM   #6
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Go to and look-up FastPlastic he/she has done some nice aerodynamics work with his/her XJ in addition to other MPG mods. FastPlastic has also discussed some very promising ideas that he/she has not yet implemented. You'll have to do quite a lot of work to get 25+ MPG, and you'll find that a WARM air intake will be more beneficial in this regard than cold.

With grill covers, stock to small lift with lightweight belly pan for reduced undercarriage turbulence, keep the air dam, warm air intake, no armor (keep the vehicle as light as possible, roof rack delete, electric fans, quality tune-up, quality O2 sensors, stock-sized tires, no AC (even better if you have the guts to remove the AC compressor completely)... You can hope for MAYBE 25 mpg highway. I'm using 4-hole injectors, but I wouldn't call them an MPG booster.

The 4.0 can be improved a little bit, but it's not the most efficient engine, and there isn't a ton of room for improvement. The AW4 has goofy shift points that certainly don't help either, but not much can be done there other than to swap-in an AX15 manual or something. Using a 2wd XJ would also help quite a bit. The XJ's aerodynamics are the real enemy, and where the biggest gains can be had if you have the fab skills.
Build Thread --
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Unread 07-21-2013, 04:20 PM   #7
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The TransGo kit can help with the AW4 shifting. Lose the passenger side mirror and radio antenna. Push the wheels out where the tires are flush with the fenders. Flat wheels help. Run minimal fender clearance. Pull the front driveshaft when not needed. Trade out your 300 cpi cat for the shortest one you can find with a cell count around 100 cpi. Vent the hood (especially across the rear) AND vent the fenderwells into the engine compartment. Isolate the timing (via Msd) and play with advance. Drop pulley ratio on alternator and P/S. Mod the MAP- I like the Apexi controller. This is less beneficial on OBD-II systems.
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Unread 07-21-2013, 08:22 PM   #8
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1999 XJ Cherokee 
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I did remove rook rack and running boards. Edited in later: Later on I also removed a winch and winch plate (which greatly improved aerodynamics and reduced weight 90 lbs).

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both gas and fuel economy.

I'm also considering having intake and exhauat ports ported and polished, and high flow headers (insulated with ceramic paint) and exhaust.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a V8.


Edited in Later: I didn't have intake or exhaust ports ported or polished because it was to expensive to be worth it for the return I'd get in gas mileage.

I used a Dynomax 2.25" cat back and it's great for performance, gas mileage, and is almost as quiet as stock. No noise when driving easy on the gas pedal (i.e. - when driving normal), but it has a slight high performance growl when I get on the gas pedal and the performance boost is quite noticeable above 2500 rpm. Below 2500 it has as much performance as stock exhaust (i.e. - no loss of low end torque). My Kolak ignition noticebly increased my low and mid rpm torque. So it's really quite torquey at low and mid rpm from Kolak ignition, and quite good horse power at mid and high rpm from Dynomax 2.25" exhaust. I did one of these mods at a time. So I know exactly what the effect(s) of each of these mods was.
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Unread 07-21-2013, 08:39 PM   #9
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2006 TJ Wrangler 
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Location: Baton Rouge, Louisiana
Posts: 722
The headers wont help either significantly- you already have tubular exhaust. The head work helps a lot but to realize power gains you have to change cams, and there went your mileage. In other words, theres no free lunch. You can do one or the other. But 25mpg on 30s in that 4wd while increasing performance is an unrealistic expectation. You cant have your cake and eat it too.
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Unread 07-21-2013, 08:56 PM   #10
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2006 TJ Wrangler 
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Originally Posted by Charley3 View Post
I did remove rook rack and running boards.

I am after more gas mileage and power. So my 1st priority is anything that increases both. My 2nd priority is things that improve one without losing the other.

So far I've improved both fuel economy and power.

I'm considering having intake and exhauat ports ported and polished, and JBA high flow headers (insulated with ceramic paint) and high flow exhaust. I think that would gain some more fuel economy and power.

To have a high flow muffler that's quiet, I'm thinking a stock muffler from a 300 cu or 5L V8 would be quiet and high flow on a 4L.

I'm using mid grade gas. That was the case before and after the mods I've done so far.

I think I'm going to have the fuel map sensor cleaned or replaced next.

What about EGR valve? Do 99 4L have that? Would it affect fuel economy?
A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.

There is no EGR on your 4.0l.

What on your map sensor are you going to clean? Until the much later years in the Grands and Wranglers where they were mounted on the throttle body, the map sensors sit on the firewall and gets manifold signal via a hose. They generally don't get dirty as long as the tiny reference hole isn't plugged.

Mid grade gas, unless need to counteract preignition or detonation is not beneficial. Within the same type, style, brand etc., lower octane fuel has more btu energy and will achieve more horsepower and mileage than a higher octane rating.
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Unread 07-21-2013, 09:13 PM   #11
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Join Date: Apr 2012
Location: Racine, WI
Posts: 2,602
Indeed, 87 octane is all you need. Save your money.
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Unread 07-21-2013, 09:15 PM   #12
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1995 XJ Cherokee 
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Location: GRAND FORKS, North Dakota
Posts: 87
It's a Jeep......if you want good mileage get a Dart.
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Unread 07-21-2013, 09:24 PM   #13
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1999 XJ Cherokee 
Join Date: Mar 2013
Posts: 4,352
I just got home from a road trip.

I gained another 1 mpg from high cooler flow air intake, and noticeably more power on hills.

So now 22 mpg highway. Pretty good for 30 x 9.5 R15 all terrain tires.

I will switch to 87 octane gas and see how it does. Could be a real economy boost considering cheaper per gallon.

Thanks for suggestions.
Warning: I often edit my posts a few times to get them complete, or to correct errors.
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Unread 07-21-2013, 09:26 PM   #14
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1999 XJ Cherokee 
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Originally Posted by Newtons3 View Post
A stock muffler will be a waste of time if you are looking for better flow. You will need a performance type.
I meant a muffler from a V8. That'd be high flow on a 4L wouldn't hit?
Warning: I often edit my posts a few times to get them complete, or to correct errors.
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Unread 07-21-2013, 09:30 PM   #15
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Not necessarily. There's no reason a muffler of the same type with the same size tubing will flow more air with less restriction.
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