Hey this a question that I am actually qualified to answer. I have been getting about 14 to 15 MPG with the 4.0 on my TJ. Of course this is almost all city (less than 3 miles each trip) with a few highway trips (40 miles each week). When traveling to Michigan this summer I got 17 MPG on the highway.
One of these days I will get to the Badlands, Silver Lake Sand Dunes, the new Rockport Off Road Park near St Louis or somewhere I can actually get muddy.
00' TJ w/ 33's, 4.56 gears and 5 speed. i get about 250 miles to a tank of gas.....it was worse when i had the 4.10's......hope that tells you anything thats about 13.15 miles to the gal..........not much better then when it was stock i think.
'95 yj -- I6, 5 spd. -- 255/70/15's which is metric equivalent to 31x10.50's. fresh tune-up got me back to 19 all the time. I have found that if I stay between 55 and 60 it stays there. Over 60 and it drops.