Starting with the tractive energy requirements of vehicles over prescribed driving schedules, the fuel economy potential of idealised heat engine/flywheel hybrid vehicles was established as a function of vehicle characteristics and driveline efficiency. Both passenger cars and busses were considered in this analysis. The calculated idealized hybrid fuel economies were compared with cur rent production vehicle fuel economies and the potential for substantial fuel economy improvement was noted.When the hybrid vehicle is penalized for the initial flywheel-charge energy, the potential economy gains are reduced. The effect of the initial charge energy should not be neglected in passenger car driving schedules. However, the effect is minimal for the urban schedules typical of bus service.