Bearings are already quite good. You could get the same result by using magnetic, no contact wheel bearings at a very large increase in cost, but the fuel economy improvement would be tiny.

The big problem is not rolling resistance, its wind resistance. Wind resistance goes up by the square of the velocity increase....double the speed and you quadruple wind resistance. The second biggest fuel consumer is the mass itself (not the weight). Accelerating and decelerating mass requires energy. Magnets might make less "weight" but they would make more mass.