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!