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Unread 03-10-2009, 12:30 PM   #1
NaeKid
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FAQ - My gas mileage sucks

Every Jeep owner asks himself this question - "What do I have to do to get better gas milage?" Soon afterwards the question shows up on JeepForum. This usually happens in the winter-time. So - I figure its time to create a FAQ on why gas milage sucks worse in the winter than in the summer.

First things first, I will assume that your vehicle is tuned up with good spark-plugs, clean filters (air, fuel, oil) and that all the fluids are changed at the recommended intervals.

Second, I will assume that the air-pressure in the tires is at the recommended PSI and that the tires are rotated regularly (not by vehicle power, but, actually removing the tires and putting them on different axle-ends).

Now, we will get to the meat-n-potatoes of the question and what you can do to get either better gas milage or possibly just make it suck less.

Remove all excess weight from the Jeep. Do you need to carry that dead-weight of a winch all the time on the front? Do you need that dead-weight in the back of ropes, chains, axes, hi-lift, spare-tire, tools, fluids, back-seat, etc. Is it better to try to improve your aerodynamics by removing the huge rack over the roof when you don't need it there? Do you need the ski-rack on the top all week when you only go out for the weekend?

Will a CAI improve your milage? Will a snorkle improve your milage? Will any bolt-on "power improving" gadget improve your milage?

The answer is "not really" or "not significantly". Over the long-term you might gain a few miles or kilometers per tank in a highway driving situation, but, for standard city / in-town driving there really isn't any significant improvement due to factors beyond your control. Street-lights on every corner stopping you, idle-time, traffic-flows during rush-hour, etc. Whatever gains that you might receive from the gadgets would be negated by those kinds of circumstances.

If you are a coastal driver, you have the "thickest" air. Adding any goodies to try to improve milage won't do you an ounce of good. If you are a mountain-driver, you will have the thinner air. Anything that you can do to force more air into your Jeep will help - a bit. When my Jeep is around the coast, I have had ~600km/tank. When my Jeep spends its time at higher altitude (my normal region), my gas milage drops to ~400km/tank. This shows what kind of effect the different areas have to an engine.

Gear-ratio.

If your Jeep came with the 3.07:1 gear ratio, you are running the best gear ratio for highway. It will be a little harder for you to get off the line, but, your RPMs will be lower - saving you gas, but there is a greater chance of stalling out on the trails.

If your Jeep came with the 3.73:1 gear ratio, you are running a decent ratio for street / trail use - neither will shine - neither is very detrimental to driving.

If your Jeep has 4.10:1 gears (non-4-cyl) -> 4.88:1 (or further) then you are raising your RPMs (and fuel-usage) at highway speeds, but, it also gives you greater off-road ability.

Finally - lets get to the weather-factor that ruins gas-milage.

When the temperature starts to drop below freezing all the fluids in the Jeep slowly turn into a gel. You can experiment with this at home. Take equal amounts of engine-oil, transmission oil and gear oil and split them up.

One container of each will be kept at room temperature.
One container of each will be placed in the fridge over-night.
One container of each will be placed in the deep-freeze.

Gather up some containers to catch the fluids and set them out. With a friend (or more) take one batch of similar fluid (lets say engine oil) and try to pour them all out - start at the same time and the container that empties first is the winner.

Repeat with the other fluids.

What you will find is that the warmer the fluid is, the easier it will be to pour. Consider this fact when you think of the metal parts in the Jeep trying to move through the fluid. Your engine needs to work much harder just to get the metal moving through the fluids - then try to get the Jeep moving as well.

Finally - cold-air is dense air. Meaning that the colder it is, the "thicker" the air is. When it reaches the engine, the heat will expand the air. You are then "pumping more air in" than the thinner summer air. This is significant due to more air in = more gas used. The fuel-injection-system recognizes the air-to-fuel ratio and adjusts as required. The CO2 sensor in the exhaust tells the computer that there is more air or more fuel or more un-burned fuel or ... and compensates to a factory-set "perfect ratio".

I am sorry - but - with all the factors working against us, getting better gas-milage is difficult at best. There are little things that you can do - lower your speed, change fluids and filters regularly and remove any excess weight.

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Unread 12-01-2009, 07:21 PM   #2
rhcpfiasco10
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... awesome
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Unread 02-18-2010, 10:55 PM   #3
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NaeKid, I like the write-up. It is pretty informative. I think I will just start point people here instead of writing it out every time.

I did notice a couple of things that are missing. One big thing that affects mileage is how people actually drive. It would be surprising to hear that I get about 23-24 mpg (last 2 times I calculated, and they were in ~60 degree F weather).

A big thing I noticed is that most people accelerate incorrectly. I don't understand why people think that getting to speed slowly is better for mileage. I am not talking about racing to 55mph, but running slowly keeps the motor in the higher RPM's too long, and it doesn't reach stay where it needs to be for efficiency. Another big thing I have notice from watching my sister and mom is that people have terrible pedal feel. When starting, a many will be jerky on the throttle. When holding speed, they can't be smooth enough with the throttle, so they feather it. When slowing down, they don't maximize the time off the gas. I read that any vehicle built after '93 uses 0 fuel when coasting. Mythbusters showed in the tail gating special that feathering the throttle actually decreases efficiency.

I guess I just wanted to bust out that bit of info. No mileage thread will ever be able to be complete and cover every single model or setup that this forum sees.
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Unread 02-22-2010, 08:14 PM   #4
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Aren't all your fluids warm when the engine is running?

Certain things will help gas mileage...so don't say it can't be done, is there a quick fix to make a vehicle get 25MPG instead of 15? No...

A snorkel will decrease gas mileage, the longer, rougher, or smaller ID any pipe is causes more friction with the fluid flowing through it (Harder to pump or suck through it)
Cold Air Intake will get you better gas mileage with its smooth walls and large ID, not to mention larger surface area filter. Same idea with exhaust, larger ID exhaust with mandrel bends will flow better, using less power to push gases through. Take it further with ported and polished heads, look at a Hesco head and tell me smooth, larger bore piping doesnt flow better, they net 30 hp...and yes that will equal MPG's if you aren't on the gas all the time.

A programmer can get you better MPG
Free spinning hubs
Fuel Catalyst-(or so I've been lead to believe, haven't tried it myself)

But all that other stuff about weight and roof racks and filters is true too
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Unread 03-29-2010, 04:37 PM   #5
WranglerLegend
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its impossible to use 0 fuel when coasting, your pistons are still moving, fuel still running through the injectors. but i do have a question...what is the average gas mileage of a 2001 wrangler? mine is horrible, and i drive fairly passively. i think its supposed to be around...what, 18? i get 12. everything is up to specs and all that. or better yet, does taking the top off reduce MPG? i totally didnt even think of that until now...
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Unread 04-21-2010, 03:12 AM   #6
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WranglerLegend...came across this post today, saw no one replied to you...your mileage issues come from your top being down year around. My Jeep (95 YJ, 2.5L) will see significant decrease in mileage when the top is down. The air you're pushing through doesn't just go around the Jeep (already aerodynamically poor), but curls around the open top and spins around. I notice my acceleration is worse, my gas pedal is down farther than usual, and my gas gauge drops even faster than usual (unbelievable, I know) when my top is off.
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Unread 04-23-2010, 10:02 AM   #7
WranglerLegend
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oh thanks a lot! i figured that might be the case but was just checking to see if anything could have been the problem. i also have an air-raide intake which was supposed to improve gas mailage. it kind of did, but not really. im still dropping 60$ everytime i fill up.
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Unread 05-06-2010, 03:15 PM   #8
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you may try advancing your timing. 2006 distributor isn't in a fixed position. remove cover, scribe disk and inner wall of distributor and move 1/16 of inch in advance direction. It seems to help and notice it more between shifts. P.S. when milage wears distributor gear down some and timing chain streches with age it helps to get back some timing. Helps keep piston pressure at top dead center and motor running smoother. On fix distributors file pick up hole and make it a adjustable slot and move distributor pick-up does same timing trick good luck and enjoy
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Unread 06-04-2010, 09:37 PM   #9
mtsnowking
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Ok so i have a problem, i just put 33x12.5 tires on my jeep with a 4inch lift and i get like 11 mpg. haha its so bad,but i was wondering wht i could do to get that a little better! before my stock jeep got liike 15 i think
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Unread 06-04-2010, 11:36 PM   #10
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Gears. Fill out your profile so we know what we are working with. I am willing to bet the problem is twofold.

1. Your speedometer is off so your miles traveled is incorrect.
2. Your Gear ratio is far from ideal. That makes you engine lug and work harder. Post a thread in TJ Tech so we can help you out.
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Unread 06-07-2010, 12:26 PM   #11
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Quote:
Originally Posted by WranglerLegend View Post
its impossible to use 0 fuel when coasting, your pistons are still moving, fuel still running through the injectors.
It is not impossible to have 0 fuel when coasting. Most modern ECU's use what is called "decel fuel cut" which will completely cut the fuel flow to the engine during deceleration. I do aftermarket standalone fuel systems regularly and you can even see on the AFR gauge that it will spike completely lean on decel when you have it setup to do so. I have not calculated the actual difference in having this feature, but it is said to make a decent impact on mpg's.

Are the stock fueling systems on these pretty well tuned? Or are people seeing good power/efficiency gains in switching to programmable systems?
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Unread 06-07-2010, 12:58 PM   #12
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They are well tuned, and they do take advantage of the coasting thing. You can watch my mom's van onboard computer go to 99mpg real time when rolling down a hill. 99 is the highest the number can go
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Unread 06-10-2010, 11:47 AM   #13
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Has anyone actually taken the factory roof racks off? And if so was there any MPG gain. I remember readiing somewhere that it would be 1-2mpg better on the highway. However I really do not know if I beleive that.
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Unread 07-01-2010, 09:10 AM   #14
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I installed a Kargomaster Kongo Cage on my 03TJ with thule cross bars .. best investment I could have added to the Heep. I can haul almost anything around now, and the 500lbs cargo capacity means I don't have to worry about something being to heavy to haul. It is basically a ford-ranger flatbed on my roof!!! Except that it is 2 meters tall, so putting anything up there is a chore.

Also, I have only noticed negligible fuel economy loss (only about 5km/tank) with the rack on. Of course, once the canoe goes up and the top comes down, then the economy sags!
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Unread 07-24-2010, 05:59 PM   #15
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i think i am getting like 12 or 13 mpg! ugh...
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