I have 4.1 and bfg 315/70 17, and is time to change them..
Wat is the best size, for cruising at 85mph and get the best mpg at that speed.
I dont tow.

 

3.42 gears. or stock 265/70R17 tires. Frankly, though, nothing is going to give you "good" mileage at sustained 85mph cruising. It takes a bunch of fuel to push 8000lbs. through the air at that speed, regardless of the gearing or tire sizes.