If it was rubbing to the extent of causing lower mpg would it get really hot??
As you say I would have thought, as with bikes, any considerable rubbing with such momentum behind it then the pads would either burn away and stop rubbing or just be forced away from the disc. However I can assure you if I jack up the rear wheels and spin them they spin nowhere near as freely as the front ones as can feel the resitance as it's turned.