Recent advances in hydrostatic transmission efficiency and accumulator technology make the hydropneumatic energy storage automobile appear quite attractive as a means of improving fuel economy. The system examined in this paper utilizes a conventional internal combustion engine, and two hydrostatic pump/motor units with an accumulator between them. The accumulator allows regenerative braking and permits the engine operation to be uncoupled from the road load. Detailed, second-by-second driving cycle simulations have been used to study the fuel economy possible with various combinations of component parameters. The design can provide excellent fuel economy with a moderate size accumulator.