Struggling with interstate mileage in 2000 Jeep Wrangler with oversized tires.
 

I've been struggling with a 2000 Jeep Wrangler that I bought used three months ago. I've climbed up to 20.5 mpg from around 17 by inflating tires, cap, rotor, wires, plugs, and switching to full synthetic lubricants in engine, differentials, and transfer case.

What I'm not sure of is why I only get around 18-19 when I take the interstate bypass to work. I drive 55 on it and baby it. When I go through town for my commute, I drive 45-50 and get 20.5, despite stopping and starting through a dozen lights. I've done this repeatedly, devoting entire tanks to each type of commute.

My Jeep has the 2.5L four-cylinder, and came from the previous owner with 31x10.5 load range C tires. Those are 4.5 inches larger in diameter and 2 inches wider in width versus the stock 265/75R15, if my calculations are correct. I'm also guessing that the 6-ply rating makes them a little heavier than regular 4-ply tires.

The Jeep has always had sluggish acceleration, and in 5th gear, it really struggles to accelerate.

So, I added a vacuum gauge. What I notice is that I almost always get better vacuum in fourth gear (12-14 inches) than I do in fifth gear (8 to 10 inches). This holds true for speeds of 45-60. Under mild acceleration, vacuum drops much lower in fifth (can drop as low as 5" or 6") than in fourth (usually stays above 10"). I lose more vacuum going up small hills in fifth and I gain back less vacuum doing down hills in fifth.

I can't be positive, but it feels like it does take more pedal in fifth, but that could just be my paranoia.

I'm wondering if the larger tire diameter is lowering the engine's highway RPMs out of its useful power band? Could that, combined with heavier tires be making my overdrive inefficient, placing too much of a load for that engine speed?

Maybe the bad Cd of the Wrangler is costing you that much at highway speeds. It was never designed with high mpg in mind. It's specific purpose is not that of a highway car. Maybe driving the slower commute keeps you closer to the sweet spot, and once you try to go above that range the poor aerodynamics add up to kill your attempts at higher mpg.

Smaller tires, just DO IT. So many people don't realize that those things are just stirring air.

Welcome to EM!

Yes, the oversized tires are bad aerodynamics, and they can't have good rolling resistance, and they are heavy! Tire weight essentially "counts twice" -- it is part of the total vehicle weight that has to be moved; and it has to be spun. The stock tire size with smoother (all-season) tread and say 44psi in them would be better on all three counts.

What is the stock tire size?

Do you need a jeep? This is a serious question... just making sure. If you need it, it may be worth considering buying an inexpensive little car with twice the MPG that you use for regular highway commuting and such and just use the jeep for what you actually need it for.

Where's the fun in that? :p

it almost sounds like you have to put numerically higher gears in the jeep to raise the rpms up on the highway to get it in it's efficient range or get smaller tires

I agree with several ^^^^ about the tires. Those are probably the main problem you are having. If you have the cash get them swapped out.

You actually want this to be the case. More load means decreased BSFC, and better efficiency.

The old EPA sticker that came on your Jeep would have read 18 city/20 hiway with the four cylinder stick. The new revised sticker that would come on it if sold new today would be 16/19. Using this as a ruler shows that you are in the ball park. As for the tires. If your Jeep did come with the 265/75/15s then they would be 30.7 inches in diameter. The 31 in tires will give you a 1% slower speedometer reading and throw off your mpg figures by that same amount. Your 18-19 would really be 18.18-19.19 and your 20.5 would be a decent 20.7 mpg.

I am wondering if you actually have a wrong idea of what the stock size tire was. All of my sources for the 2000 model list the following sizes, diameter, and I have added speedometer/odometer error in case your vehicle should those tires compared to the 31s:
Jeep SE
225/60/16 26.6" 60=69.9 16.5% off
205/75/15 27.1" 60=66.6 14.3% off
225/75/15 28.3" 60=65.7 9.5% off
Sahara
225/70/15 27.4" 60=67.7 12.8% off
225/70/16 28.4" 60=66 9% off
30x9.5x15 30.0" 60=62 3.3% off
Sport
215/75/15 27.7" 60=67.2 12% off
225/75/15 28.3" 60=65.7 9.5% off
30x9.5x15 30.0" 60=62 3.3% off

Where did you get the265/75/15 tire size from. That is DARN huge for the four banger!

As far as the width of your tire, a 10.5 is only a tenth wider than the 265 you stated that came on it stock, however....a 205 width tires at 8.1 is 2.4 inches narrower and more CD friendly.

Another factor is the wt of that 31. The first 5 tires in that size from my suppliers come in at an average of 41.6 lbs each....the 265 size tire average wt is 38.2 lbs....not much difference, but some improvement can be had going back to that tire. I am still concerned that that tire did not come on it.

have you accounted for the larger tires in your speed and distance traveled? Your speedometer is really counting revolutions of the output shaft on the transfer case. If you change any of the ratios after that point it will skew your speedometer and odometer. Larger tires means your rig will go farther per revolution of the tire making your mileage not quite as bad.

If you need to ask, you've obviously never driven say a Honda CRX or Mazda Miata - or even an old VW Bug :-)

Double check your door for the OE tire size. Those would be large OE tires for a 4cyl. I would guess in the range of 205 or 215's were what came on it.

The 4cyl. does require quite a bit of gearing to make anything happen. I would check the rear end ratio to see how bad it is. Your Dana 30 up front will go to 4.88.

I would be tickled pink if my Jeep got 20mpg

Happy Jeepin.

Don

Those tires sizes seem a lot more reasonable for a stock four-cylinder. I have probably about 20,000 miles of tread left in the current tires, but the acceleration and mileage is so poor, I may consider an early switch.

I've found a few 4-ply (standard load) 15 inch tires that are only seven inches wide and are each about 10-15 pounds lighter than the average 31x10.5 load range C tires I've looked at. I also have a full sized spare, so that could cut 10 pounds of sprung weight and 10 pounds of unsprung.

Don't narrower tires usually work better in shallow mud and snow anyway?

I believe my model came with either 3.56 or 4.10. I've been reading on 4bangerjeep that a lot of guys running 31s end up really happy going with 4.88s.

I must confess I don't really need a Jeep. Strictly speaking, I don't need to drive at all. I originally thought that a Jeep would work well for where I live. Last year, my two-wheel drive pickup and I got stuck four times (twice a friend's 4x4 pulled me out; twice I had a tow).

I had considered a 4x4 pickup for myself, but I didn't want to continue getting 14mpg. Even the little Rangers and Toyota 4x4s seem to be pretty hard on gas. And I don't want to mess with the new automatic locking hubs.

I was hoping I could get a 4 cylinder Jeep and get in the early twenties for mileage, and still have 4wd available when I struggle. I'm getting way better gas mileage than my 1995 Dodge Ram 1500 with that 5.9 liter V8. It got 14-15, no matter what I did to it, and I tried about everything.

I do plan to keep this Jeep running forever, and I've read they'll do it if taken care of. I know that a significant percentage of a vehicle's overall energy price is used just to create it.

I am honestly considering buying a cheap, second used car that gets in the high thirties for when it's dry out.

Just about anything will run forever if you take care of it.

So you need one because you don't want to get stuck? Reasonable reason for keeping it. (take note all of you in your lifted highway queens) It has been mentioned, and i'm usually the one advocating it... buy a car.

Oh, BTW, you don't have to thank every post that is decent. ;)

Do you have snow and ice, or other issues that might make a Jeep a good choice? I deal with snow regularly in New England - and I'd pick a fwd car with four snow tires over a fuel-sucking 4x4.

If you're dealing with mud or other hazards, do tell.

my wife does ok with a front wheel drive in AK. I do like having a 4x4 around for when I plan on going into the mountains, on the back roads, or don't want to shovel the drive. When the move to town is done we will both be sporting 1000 dollar FWD cars for most of our needs. We will also have a small 4x4 truck for dealing with other duties than commuting.

Well try to keep it clean since your aerodynamics are HORRIBLE and just switch back to some stock tires if you have the cash to Unless you actually USE it for offroad. Also you've got the 4 Cylinder jeep try a few small performance things so it'll hum along more easily on the freeway .

I owned a 4X4 for 10 years in Alaska and Utah. My wife and I both preferred our RWD Datsun B210 for snow driving. We were never confident the 2-1/2 ton 4X4 could stop in time, even with its studded snow tires. The 1-1/2 ton lighter Datsun stopped a lot quicker with studded tires on all four corners, a simple matter of less mass, less inertia.

Our other snow car (for my 5 years living in Colorado) was a RWD Datsun pickup truck. There were times the truck's higher ground clearance made the difference between getting there, when other vehicles couldn't. But the B210 only got stuck twice, and the PL521 only got stuck once, in the snow. There's an old saying in snow country. Four wheel drive just helps you get stuck worse. Give me a heads-up driver in a 2WD over the average tyro in a 4X4 anytime.

if my math is correct, that's about a %17 improvement. Not bad really. As other have said, areo, maybe smaller tires?

So...where are you located in IN...I'm in Kokomo

I'm in southwest Indiana, in rural Warrick County, maybe 15 miles from Evansville, and I grew up in Mount Vernon. I had a good college friend from UE who was from Kokomo.

Well, I've given this a lot of thought the past couple of days, kind of dealing with a crisis of conscience. Right or wrong, the local culture here worships pickups, muddin', and racing. I live just a few miles from a county drag strip. Buying this Jeep was a little scary for me; it's the first vehicle I've ever owned that wasn't a pickup. It's also the first I've ever owned to break 20 mpg. I'm also restoring a '78 F-250. I chose to devote a significant portion of my income to vehicles and fuel, but it's my form of entertainment. I rarely eat at restaurants or go to movies. I also own a very small house and spend less on heat/electricity than most do on cable alone.

I know that some may never feel my vehicle choice is justified. They may be right. On the one hand, before I found this forum, I felt my choice was really an improvement, but now I realize that I could have gained a lot more efficiency.

I deal with light to moderate snow some winters. Dirt, gravel, and muddy driving most of the year (sometimes by choice, I'll admit). I also pull a small trailer frequently.

I was really looking forward to open-air driving all summer in this Jeep, which is honestly the biggest reason I bought it!

I'm doing some real soul-searching. I hate the process of buying vehicles. I got 200,000 miles out of my Dodge before I traded it in. There are a lot of viable high-mileage options that I didn't consider before I bought this Jeep. I'd at least like to try this Jeep for a year (I just registered it and paid the state sales taxes). But in the meantime, I'll research other efficient, robust options.

At the very least, I'm fighting to make the most of every drop of gasoline I burn. If I hadn't bought this Jeep, some other guy would have, and around here, the odds are pretty low that he would care about fuel economy.

I do genuinely appreciate ya'll's willingness to advise me, even if my vehicle choice isn't ideal!

awell if you live somewhere like that and you're dedicated to your jeep it's a great choice especially over a pickup just follow the tips from guys oh here and you should see your mileage go up and up slowly but surely

Good luck

I would recommend light weight wheels and smaller tires. I have a 2002 with the 4.0 and in mixed driving I am at 15.6 mpg. I run 225/70/15 and I want to get lighter rims and switch to 215/75/15. I just switched to a double platinum plug and changed out the air filter. I had the oil changed and replaced with synthetic. I also feather the pedal and take my time. I want to get over 20 in mine.

A heads up driver in anything will beat a tyro in anything. It all comes down to not out driving the capabilities of your rig regardless of the conditions. Or as was better stated "a mans got to understand his limitations".

I've got the perfect solution for you. Sell the jeep in a year, and if you really like the open top, get a suzuki samurai! I just traded for one for my hypermiling homestead project vehicle that I've posted about here. I got 31.54 mpg with it on the first big 260 mile road trip with 4 mountain passes to go over. Mine is a carbed model with "oversize" 225.75.r15 tires that the previous owner put on. I am also 6'5" 230lbs, another myth about the big guy needing a big pickup :-)

haha Better yet, get a suzuki x-90! lol

http://upload.wikimedia.org/wikipedi...i_x-90_red.jpg

I always thought they look like a loony toon that just got bopped in the head with a hammer and has a growing bruise. Like a lot of suzuki stuff, ugly as hell but small, dependable, and easy to work on... I dare you to drive an x-90 around. Doing that definitely takes a bigger man than any kind of oversized tyro rig.

I would second a samurai for a fuel efficient off roader. The ones I have owned have been unstoppable. I have pulled a couple full size rigs out of ditches, and yanked one out of snow up to his axles he broke through a crusty layer and I just floated atop it(He hung his head and asked me not to tell anyone...hehe)

An x 90 is the same drive train and chassy as a Suzuki sidekick/geo tracker. It is very stout for it's size. Though a swb tracker has a great deal more room inside than an x 90 I have seen x90s with a bunch of lift, lockers, 31's and bull bars with winches. Actually looked kinda neat.

I was thinking samurai so that he could have a soft top still, something he mentioned was his main reason for getting his jeep.

The samurai weighs so much less than a wrangler that I bet the power to weight ratio would be similar to a 4cyl wrangler.

Oh, and that 31.54mpg I posted for my last samurai trip was to drive it home after I traded for it, so that is without a tuneup/ airfilter change/ etc. :cool:
and it is using google maps for distance travelled since speedo cable needs replaced, so no tire size /mpg inconsistency. take that jeep wrangler :-)

4 cylinder jeeps have 4.10 gear ratio, stock; does your jeep have a lift of any kind on it? have you checked to see if your speedo is accurate?