Start with a tune-up. Change your spark plugs (I used XP985's), rotor, cap and wires. Consider changing your engine oil, transfer case and diff. oils over to full synthetic. Have your transmission flushed (I have mine flushed by a professional). Consider a Sea Foam treatment to clean your fuel injectors, also clean your throttle body as stated above. Later this spring, flush your cooling system (Won't really effect mileage, but probably needs done.)
You should be able to easily do all these yourself with basic tools. This forum and Stu Olson's will have write-up's on how to do all of the above if you need them. Good luck, and enjoy your Jeep!
I also have a 99TJ with auto. 3.73 gears, 31x10.5 bfg's, next month going to 32x11.5's. I can sweek out 240 miles on a tank, highway driving and get about 120 miles out of a tank in town. Thats running it down to empty. Ive done the tune up, and cleaning with no avail. Remember, its a JEEP. The 3 spd autos will always get poor milage over the standards. The poor milage is always out wieghed by the coolness of being a JEEP owner.
I have always had big trucks and I have a lead foot too, but my jeep runs great with no problems. She a happy jeep at those speeds, and she just purrs along the highway.
you and I both. I don't mean to drive it at 75-80. Hell once I hit 90 but I didn't know it. It just barely starts to vibrate. It drives like normal at 80. At anyrate I drove 65-70ish last time on a two hour trip on the highway. Got much better gas milage
I have to agree with KenG, I have a 97 4.0L auto, did all the tune up new plugs wires cap rotor and I get about 120-130 to a tank. That is to work and back apx 14 miles no big open roads on the way but owning a Jeep is cool, wish I had bought one years ago...next new ride will be another Jeep
I just picked up a 97 2.5 auto... Put a set of 31s on it with no lift.. Tires only measured out around 29 inches... Check my milage after driving some hwy and city combined and was averaging around 14-15... This is with a cracked manifold and needs a tune up.. Planning on a Tune up, change fluid in tranny and fresh lube in differentials.. This should give me a little more along with a smoother running engine..
For those getting bad milage, make sure your speedo is correct and try to use same gas pump pointing in same direction.. Always use trip meter, never try to use fuel guage when figuring.. Mile Markers are the best way.. I always heard 31s make your milage off around 8-10 %... When I go a mile on freeway, my trip always says .9 everytime..
Shoot, my Jeep hit 70+ on my first test drive. Most of my commute is 40-55 mph country roads with a bit of stop and go on one end of the commute. I do get on it pretty good but, I am still averaging over 15 mph with a 4.0 6 spd. @ 45K.
I hope you are able to fix your mileage woes soon.
Folks loves us some Jeeps out here.
I have a 99 TJ with 140K. I get between 17 - 19 mpg depending on how much highway I get.
I also almost never go above 65.
I do 70 occasionally but only when behind a big rig. I have a soft top and you can really tell a difference when you get behind a big rig. I probably have annoyed a few truck drivers so far, but hey gas is expensive ...
I have a 99 with 4.0L and 5 speed with 95k miles. On the highway I get about 18. I towed my 15 foot cargo trailer over a 5,000 pass this weekend and got 12.3 mpg. It really does matter how and where you drive.
99 Sahara - 5 speed swapped out to automatic
12,500 lb Winch, synthetic winch line, Spyder Front Bumper, Aircraft Lights
Rancho 2.5" Suspension Lift, 1" body lift.
285/75/16 Guard Dogs, Level 8 Alloy Wheels
4.56 Gears, Dana 44 Rear, Spartan Locker in Front, Powertrax No-Slip locker in the rear