well moving up any at all, in width or diameter, will cause some change in mileage...more mass for one thing so you cant get away from that. it might not be enough to cause any concern but you cant expect it not to go down any at all. FWIW i went with the 275/70/18 toyo open country ATs and love them. they are an LT 10 ply load range E tire and are stiffer but they are great in snow or dirt or mud so far. and i might have lost .2 mpg at the most....but expected to since i changed from stock size.