Expect to lose mpg with those 315s. My truck came with those in BFG brand, and while they looked great, I didn't realize how much power they robbed until I swapped them to 285/70/17 Nitto Dura Grapplers. Truck would coast twice as far and I picked up at least 2 mpg on the highway. And yes, I did use my Smarty to allow for the different tire diameters.