Even if you stay with a more narrow tire, like a 235/80R17 or 255/80R17, your mileage is not going to improve, and likely it's going to get a little worse. The only way taller tires can improve mileage is if you have something like an M1008 GM military truck, where the factory axle gearing is 4.56 and there isn't an overdrive, so at 55mph the engine is running 3000rpm's. Your truck isn't like that at all, and so taller tires just make it work a little harder to go the same speed. They will slow the engine down for a given speed, but on a modern vehicle, that's not a good thing, since your engine is probably already spinning well below torque peak when you're at cruising speed.
On another note, I'm running (~33") 255/85R16 Maxx's on my TJ and I LOVE them. (255/80R17 is the same width and OD) Maxx's are pretty spendy though... If you can live with a 32" tire, you can get really good pricing on AT3's in 235/80R17, and they seem to be excellent tires too, and would likely give better mileage than the Maxx's.[/
Thanks Deuce, will the 255/80R17 32" work with my 3.55 gearing? My current tires are 235/75R17. Will a 31" Or 32" affect mileage that much? I'm not looking for additional width, just added height.
I used to own a 1986 M1008. I ran Cooper 10 plys on it too. It hated going faster than 60. Sad to see her go.
I've looked at and attempted to understand how the numbers on the sidewall relate to inches, height and width. Mathematics is by far my worst subject.........