Skater28

its a 94 XJ 4.0 auto. I get 8-9 MPG city about 15 highway. Originally had 235X75X15s now I have 31x10.50x15 on stock wheels. Milage only changed about 1 MPG after tire change. I have a light foot. DD only. Any suggestions? Jeep runs great, no rich smell,or smoke. Passes NY inspection with out a problem
Thanks Frank

Jeepman659

What most guys on here will tell you is check fuel filter injectors air filter spark plugs etc. and bigger tires hurts it alot. the way to compensate is to re gear but it wont bring it back to stock. also Ford 19lb injectors off the mustangs or the 2.4l Neon injectors are supposed to help out alot (still in the process of buying the neons) There's alot of options just depends on the money.

cdeal28078

Are the Ford injectors out of the 3.8 V6 engines? I just pulled a 3.8 out of my daughter's 96 Mustang and replaced it with a low mileage engine due to a head gasket failure so I have all of the parts left over including the injectors.
I'll have to change them out if the are the right size
clint

Super013

I hate to say it, but if every thing is in working order that may be just the way it is. the jeep I6 is not really a good city motor, and the fact that it is a 94 doesn't help. I pretty much did the same thing on my 93' XJ i went from Kelly 235-75-15 to Dunlap 31's and im almost identical to you with about 9-10 mpg in the city and 16-18 on the highway. my guess as the the slight difference is that im only 2wd.

On a side note the fact that you still have the factory rims plays a huge part in this. for about 6 months i ran some off brand aluminum rims and it was a HUGE difference. But i didn't like the way they looked so i went back to the factory 5 spoke

I'm all for the Cherokees but i have never seen great gas millage out of em'

Jeepman659

I'm pretty sure there out of the bigger 5.0 engine. if they're orange there probably the same ones there supposed to be color coded Yellow Red or Orange i believe.

Tural

that's not entirely true. i get 22mpg highway on one of the heaviest 31's out there. 68lbs per tire and wheel.
1. did you figure in the speedometer error of the bigger tires? you are driving 1.1 times the miles showing on your odometer. example, you have driven 200 miles on 31's..... you have actually driven 220 miles. hence, if you calculated 15mpg, it's actually 16.5mpg.
2. i have heard from numerous people that sometimes the O2 sensor can go bad and not throw a code and it will cause a bad gas mileage.
3. you could have a break pad or shoe interference that you're not noticing much. check your rotors and drums and see if you have any unusual wear.
4. tire pressure is very important on vehicles like XJ. they have torque to cruise with good mpg but as soon as you give it gas, your mpg gauge dips down to low teens.
5. proper amount of fluid in motor, transmission and differential. there is a slight chance that by overworking the motor with bigger tires, you have worn something down in your transmission or rear end. that is highly unlikely, but possible.

Jeepman659

very good points. especially the 02 sensor the first one actually adjusts how much fuel is coming into the engine so even if it's clogged it could make it quite a bit worse. even bad TPS sensors can have an effect because the ECU might not really know how wide open or closed the throttle body is.

antibiotics

Just to share the MPG of my 99 XJ, I6, 4WD, auto:

When I first got it, it got around 14 mpg for city driving, which I think is normal as my commute is a 2 mile commute with several stop signs and traffic lights. Then I start refresh my rig, after a major tune-up, spark plugs, wire, rotor, cap, O sensor, TB cleaning, fresh fluid in drive train, you name it. The XJ ran very smooth. However, the MPG dropped to 10
I have been bothered by this MPG for quite a while and don't understand why a tune-up can hurt MPG so badly.
About one week ago, I swapped in a set of yellow four hole Ford injectors. With a slight boost on the performance, I saw my mpg goes back to 14.

tomcarey88

poor gas.....an ethanol

antibiotics

May I ask what pressure you use?

Tural

oh dude, i drive on 10ply firestone destination A/T's. they take 55lbs minimum.

Tural

million times yes.... cheap gas will cost you a lot in FI cleaner alone. i buy beans from Kroger, not gas... (I make the gas by eating the beans.. ha ha.)

i only get gas from BP, Amoco, Chevron, Exxon and Shell... I'm hearing that BP is about to fall off my list. Chevron used to be by far the best. and it's still better than the other ones. i run it in my sportbike.

Jeepman659

Yes sir i'm a Shell man. and most common 3 ply tires it's around 33-35 psi.