Last weekend I put my 15" winter wheels/tires on and wow! I've gone from averaging 35 mpg's to 39 mpg's over the past 2000 miles. Yes I know the smaller wheels and tires have a smaller diameter and aren't really taking me as far as my stock 17's and have calculated the distance I'm going is 3.4% less now. I've taken this into account and adjusted my ultragauge appropriately to get 39mpg. My winter tires are on steel wheels.