I have a 1999 Cherokee 4.0 with 31x11.5 BFG A/T's. I am getting a steady 12mpg and theres no way it should be that poor. I have 130k miles with the following information. Plugs/Cap/Rotor/Wires all changed about 100-105k miles. Thermostat done 5k miles ago. I have a cracked exhaust manifold between the front two pipes which was welded mostly but still leaks. I run warm and the E-Fan seems to kick on an awful lot. I leak a little bit of anti freeze which I believe is the water pump since its stock but am not sure. The oil looks fine. The exhaust is in good shape minus a slight leak in the muffler. Any ideas? The exhaust smells a little rich but that could be in my head. No CEL codes either.

 

Have you changed your upstream o2 sensor? Its due and can cause poor mileage. Just had a buddy with a 99 TJ having mileage like yours and replacing the o2 bumped it over 15.

 

O2s were changed 10k ago

 

Cracked header will cause it to run rich and kill your mileage.

 

its a hairline crack that was mostly welded up though

 

Cracked exhaust manifold and bad driving habits can knock you down to 12mpg no problem.

 

With my driving habits I get 14mpg average with 4.5 inch lift and 33s

 

You didn't say if you were running stock gears/speedometer gear or not in your post and will reflect in the mileage along with all the other items.

When I increased my lift from 3-4 1/2" and changed the 3.55 gear to a 4.56 along with correct speedometer gear and running the 33x12:50x15's. The fuel mileage increased using a GPS both before and after it went from 10.3 to 15.6 all highway and 14.4 overall.

Well worth the investment of less than $500 for the gears and master install kits (total for both sets of gears). The increase in mileage and overall performance was very noticeable and undoubtedly one of the best upgrades that I have done to my XJ along with the Lockers, which were done at the same time.

 

Are new gears necessary for 31" tires and a 2.25" lift? I have a stock speedo gear, im not sure the axle ratio but its a sport model with an 8.25 in the rear

 

Auto - 3.55
5spd - 3.07

Even with 31's you should be getting better than that. I suggest replacing the exhaust manifold, you saw one hairline Crack ... Ten to one says you missed a few. Last manifold I had welded up had 6, at first glance I saw one, couple days later I noticed a few more.
Shop welded and gusseted the manifold ($40 g so worth it to me for now).
Walker make a stock SS replacement with expansion tunes on it that stops the cracking, have one on my 2000 (93 head swap) And it works fine. Still getting 16 highway (35s 4.56 auto). Stock MJ pushing 235s seeing 22 highway. You should be in the 18mpg easy.