I have 35x12's on the factory 20" wheels and it has certainly affected my mileage. The truck is stock other than the Fox 2.0 level kit and 35" tires. When the truck was bone stock it was fairly easy to get 23-24mpg. my current situation always ends up being around 18-19. That said, I've heard other folks say they are getting 25-26 with about the same setup. Maybe its the Minnesota fuel, don't know but I'm looking for ways to get some MPG back.