1998 TJ, Poor mileage with the 4 cylinder

r7955 · #1 (**permalink**) 11-02-2009, 07:39 PM

Hello,
I've tried finding an older post that is similar to my problem, but no luck.
So please bear with me.

This last Sunday, I just bought a 1998 TJ with the 4 cylinder 5 speed, 108k miles, great shape for being 12 years old. It has 31 inch bfg at's on factory aluminum "gambler" style rims, with no lift. Jeep has no ac or cruise, drives straight as can be, with good power.

I filled it last night, have driven 31 miles so far today and have used 1/4 of a tank of fuel. About 24 miles city & 7 highway. I don't drive like a speed demon (40 in town, 60-65 on the highway).

Any ideas why I'm getting such poor mileage? Like I said, good power out of the 4 cylinder, pretty tame tires (proper inflation). But I will not be able to keep it if I get such horrible mileage. I'd be looking at 120 miles to a tank of fuel.

Thank you for your help.
Randy

carnuck · #2 (**permalink**) 11-03-2009, 12:12 AM

It may not be bad fuel economy, but a weak gauge or you have a neighbor with a rubber credit card (garden hose)

r7955 · #3 (**permalink**) 11-03-2009, 08:23 PM

I like the rubber credit card saying. I've never heard that before now.

Bounty__Hunter · #4 (**permalink**) 11-03-2009, 10:05 PM

Could be a number of things.

#1 make sure the speedometer has been corrected for the oversize tires. This requires using one of the many online gear charts to determine what the correct speedo gear is for your tire size/axle gear ratio combination. This will send the proper signal to the ECM for good engine performance and also beneficial in calculating mileage.

#2 the engine is lugging and not operating the in the peak power range after the installation of oversize tires. The only correction for this is to regear the axles to a lower gear (numerically higher). Your stock axle gears are 4.11 and switching to 4.88 would be a great improvement, even allowing you to run 33" tires someday.

I run 35" or 36" tires on my 2.5L 5spd jeep and get 17mpg, a good number considering tire size and wind displacement of this beast. The 5.38 axle gears are primarily responsible for this good mileage as it keeps the engine operating in its peak power range.

cali_92 · #5 (**permalink**) 11-05-2009, 11:53 AM

idk what the problem is. check fuel lines idk. maybe small leak

my 2.4 with 31.5" only gets about 21 miles/gal i i drive real easy
its the box style of the jeep