Like the previous post state, it all depends on your drivetrain. I run 4.88's in my TJ with 4.0/5spd. I run it all 3000 miles out to Moab and back though the Rockies and I average about 16mpg for the whole trip. Not to say the engine isn't working since at 70mph I'm taching 2950.
I have a D44, personally I wouldn't invest my time and money into a D35 (if thats what you're putting the gears in). I've seen some guys sucessfully run 33's on a D35, but they had autos and a "very" light foot. More often then not you're gonna snap an axle shaft.
If you plan on keeping the Jeep, take some time, save the money, and just swap in a superior axle (D44 or Ford 8.8) and do your gears then. Just my $0.02
Best of luck