[awesomejack]

First, I have a 96 4.0L XJ that is only 2WD with 211k miles. Currently I get around 22-23 mpg on the highway and can reach 24.5 mpg if I keep it around 60. The main reason I'm interested in lowering it is to get slightly better gas mileage. I don't want to hear any comments like 'that's dumb, lift it, bro' or 'Its a Jeep, get a civic if you want to save gas'

As part of the plan, I would run slightly smaller tires. I have a 5-speed, so it has 3.07 gears. With the smaller tires, the engine rpm's will increase for the same speed and slightly reduce gas mileage. Do you think this reduction will negate any mpg gains due to being lowered? Would it be better to only lower it and not reduce tire size at all?

Assuming the mileage is increased from 23 to 25 mpg, I'll drive it anther 40k (on the low side), and assuming gas will average $4.00 over the next four years (again on the low side), then I'll save almost $600. That estimate will only increase as I drive more and gas becomes more expensive. But, the gas mileage may not increase that much either.

And as far as the method, I have heard elsewhere that the best way would be to use stock ZJ springs in front and do a spring under axle in the rear. The leaf springs will have to have a slight lift because using the stock springs will lower it too much compared to the front.

Thoughts?

[polizi212]

I doubt you will likely see any fuel savings that would be in anyway to your advantage! In essence these jeeps have the aerodynamics of a cinder block, and as such using smaller tires and increasing the rpm would negate any slight fuel saving you might receive by such a venture. Lift it, Lower it, drive it off road, on road or down the sidewalk its your choice since its your jeep. I just cant see this doing anything beneficial for your wallet!

[rugsucka]

Stick with stock alloy wheels, touring tires, remove the roof rack, remove all non-essential crap from the hatch, carry a donut instead of a full size spare, strip body molding, fold in the mirrors (or swap to 97+ mirrors so you can).
You could cut a coil civic style to drop it a bit, but that's pure ghetto.

Hyper mile it off the line and irritate the folks behind you.

I'm sure you can achieve higher mpg, but you're not exactly in the fuel economy section of the automotive landscape.

[Trons]

'that's dumb, lift it, bro' or 'Its a Jeep, get a civic if you want to save gas'

Seriously, though, I'm not sure how well your plan is going to work. As many have reported, lifting it doesn't seem to have a noticeable change in MPG, so I'm not sure why lowering it would. I also think your theory on the tires is backwards. We put bigger tires on stock gearing and notice a decrease in MPG, why wouldn't putting smaller tires on, increase MPG? You mentioned that you'd get more RPMs for the same speed, but if you decrease the tire size, and don't change the gear in the transmission, your speedo is no longer accurate, so...not sure, I just woke up and am having a difficult time wrapping my head around it. Maybe somebody smarter will chime in on that.

[ibjeepin95]

TJ springs will lower the front...

[mschi772]

Among some others, FastPlastic over at ecomodder.com has done some amazing things to his XJ all without lowering it or doing anything else to kill its Jeep soul since the XJ's biggest enemy in mileage is simply aerodynamic turbulence, and lowering the suspension achieves relatively little in terms of reducing drag. (http://ecomodder.com/forum/showthrea...mods-2614.html) I would love to have the time and the skills to put together a lightweight under body skid like FastPlastic's for some drag reduction and minor protection.

Beyond making it look like a non-Jeep station wagon, I also wouldn't recommend lowering an XJ much because you will quickly run into clearance issues especially with your oil pan. With 2wd and a manual transmission, I could see you getting some spectacular mileage (for an XJ). rugsucka is right about removing the roof rack (even if it's simple removing the cross bars) and getting 97+ side mirrors. I'm guessing you already have a compact spare since it's a 96, but those things would be quite easy and should help.

Keep your stock tire size. If you change your tire size too much without changing your speedo gear, the computer won't be accurate anymore, and you'll have a false mileage every time you calculate it. There are a lot of misconceptions out there about using bigger or smaller tires to get better mileage, but there's more to it than just increasing circumference or reducing rotational mass. Narrow, lightweight with low rolling resistance in whatever diameter is stock is about as good as you can do in the tire department.

[Camfam07]

wow! lowereing it would do absoluetly nothing! like everyone said its a cenderblock, no matter how low it is wont increase it, tecnically if you raise it with a front skid you will push more air under the vehicle and then remove roof rack and take out the back seat if you dont even use it, I would love love to have 24mpgs on mine! im envious of just that bro... get passenger/highway tires and call it good maybe a kn&n intake fuel filter, dist. cap and rotor proper alignment... things of that nature would get you better results! im telling you a bet there are only a few people on here getting that kind of MPG's bro!

[Durt_Diver]

edit: my mommy once told me if you have nothing nice to say, don't say it at all

[awesomejack]

Those are all pretty good responses. You guys may be right and it might not improve very much at all. I'll think about it some more, mschi's link looks like it will be very helpful.

I already don't have any roof rack (although there have been a couple occasions when I wish I did), the only extra weight I carry is about 50lbs of tools, and it has street tires.

One thing nobody's mentioned is doing the dual electric fan swap. My fan clutch is on its way out (it heats up only when its stopped, and when I turn on the electric fan it cools right back down), so I figure now's a good time to do the swap. I've seen a few people report about a 1 mpg gain without the mechanical fan.


Right now the speedo gear is off slightly and I correct for it every time I calculate mileage. My odometer reads 4% higher than actual distance traveled, so I do (odometer reading * 0.962)/(gallons to fill tank)

[mschi772]

Originally Posted by awesomejack

One thing nobody's mentioned is doing the dual electric fan swap. My fan clutch is on its way out (it heats up only when its stopped, and when I turn on the electric fan it cools right back down), so I figure now's a good time to do the swap. I've seen a few people report about a 1 mpg gain without the mechanical fan.

Yep, that should help, too. It won't be much, but if your clutch is already dying then it really is the perfect time to go electric. In terms of air intake, I'd stay with the stock. Aftermarket filters filter less, and cold air, while providing a power increase, will also reduce fuel efficiency a little. Actually, some of the really dedicated "hypermilers" have mods for partial or temporary warm air intake to increase their efficiency. Whether or not a warm air intake increases efficiency on our 4.0--I'm not sure. The way that WAI increases efficiency is actually a bit more complicated to explain than explaining how CAI increases power, and it doesn't work on all engines.