Come spring time I am putting a 6" suspention lift on top of my 3" body and 2" leveling. Was definitely set on doing 38's but since one inch isnt much of a differance I figured I might run 37's. Just didnt know if there would be a huge differance in mpg in one inch?

 

Go with 38's. If your going to have that much lift you'll want more "tire." Don't even think about the MPG factor; there's no difference.