What kind of MPG dropping am looking at with 315/70/17?

Yes, increasing tire size while running stock gears will make the drivetrain work a bit harder. But, many people have been running 35's and 37's for years without regearing, and there hasn't been a rash of posts on the engines blowing up. Regearing to a proper ratio for the tire size will of course lessen that stress and let the drivetrain perform better, but then you still have the added stress from the bigger tires to account for, and the loss of more of the already minimal aerodynamics from the lift, and... :dontknow2:

You will be fine in town. If you have done any plugging in the charts, you already know where the rpm's will be at 70 both in overdrive and with it turned off. Where you will probably find the gearing lacking will be offroad. Expect to be in 4lo more than you are used to, rather than 4hi.

There are huge threads on mpg's, your best bet would be to read through a few and realize that there are as many variables there as in most other areas involving the modifying of your jeep. No one can tell you what you will see. They can only relate what they have found, for their specific build, with their specific driving style, in their specific location, etc.

Well losing off road capability is something I do not want to do so it sounds like Ill either stay stock size or go 33. Will
l these sizes work for off roading in Ouray and Moab? I want to hit as many trails in moab and Ouray this summer so will stock mt or 33 at work?

You guys ever go out to First Wesnesdays in Grapevine? Haven't been in a while but thinking of going next week. I have a 2012 auto with 3.73. Upgrading from 33s to 35s this week and hope to have feedback on both tire sizes with this gear setup by then.

has anybody put 255/80/17 bfg km2 on their JK? How were they? did the E-rated tire make the jeep ride rough?

Yes, air down between 25-30psi (chalk test). Your conscience will thank you while traversing over sharp pointy rocks though. I saw two ProComp sidewall failures in one day which made me love my Toyo OC M/T's even more.

So lowering the air pressure will keep the ride from getting rough. will that effect the wear on the tires? I know on a car if it under inflated it will wear more on the edges

That's the point of the chaulk test, to find an air pressure with as even of wear as possible

Make sense!

only been once. I lead a Bible study at church on Wed. nights so can't make it.

Sent from some string and a few tin cans.