I have found my mileage to be VERY dependant on tire inflation. I have found for longer highway driving, if I overinflate to 45-50psi (well within tolerances of the tire, just more than Jeep recommends) I get 15-17mpg depending on terrain (not by the dash computer, but by gps miles traveled/gallons used). At 40psi, that goes down to about 13-14 and at 35psi about 12mpg. 40 and 35psi are estimates, it has been a while since I actually ran that psi on the road to check. I find it is worth the trade off for the little rougher ride to save the $$, especially since some of my favorite wheeling areas are a couple hours freeway driving away from home.

So far I am having no uneven tire wear (a sign of true overinflation) and be sure if you are going to try higher psi u stay within what the tire is designed for.