I have read many posts that say 17" tires don't get quite as many MPG as the 15" tires. I understand why the 15" tires are a softer ride (more air in them) but aren't the outside circumferences of both tire sizes the same? If this is so (which I'm not sure about) why the MPG difference?
A wheel combination (wheel plus tyre) is heavier as the diameter of the wheel goes up. More metal, less rubber. Overall diameter is about the same, though there are small differences. Heavier wheel combination, lower mileage. A second cause is the larger wheel can put more rubber on the road. This one is harder to figure, as the overall diameter is the same, but the larger wheel usually carries a wider tyre, and there are other more esoteric effects, all of which result in more rubber on the road. More rubber results in more friction. Better handling, better braking, poorer mileage.
