You guys can bash "theory" all you want, but in "reality" you are bashing physics. The tire is the farthest point on the "circle" away from the axis of rotation, so not only does it require the most energy to rotate, the required energy increases exponentially while weight (or mass) rises linearly. Also rolling resistance (usually a function of section width and material choice) also plays a large part in fuel mileage. I inquired to Tirerack about the tire weights for the size my Jaguar takes and found that there was a range of nearly three pounds from the lightest to the heaviest. In stop and go driving that equates to some very significant energy requirements not only to get it moving (fuel mileage), but to bring it to a stop also (brake wear).