I'm running a 08 supercrew fx4 with 6in rcx lift with 295/65r20 wich is about 35.5 gears are stock 3.73 5.4 motor edge tuner and am getting about 12-14 mpg in tow mode my truck pulls of the line like it did with the factory tires.

 

Is your truck 3.55 or 3.73 right now? I take my truck off road regularly and the 3.73 perform fine my only complaint is the limited slip. I will be installing a locker soon and leaving the gears. If you have 3.73 already you should be fine I agree it could be better with a 4.56 but unless you tow regularly or do some pretty extreme wheeling I dont think it's justified. Just my opinion from my experience with my truck