The use of a hydraulic drive system with accumulator energy storage has the potential of providing large gains in fuel economy of internal combustion engine passenger automobiles. The improvement occurs because of efficient regenerative braking and the practicality of decoupling the engine operation from the driving cycle demands. The concept under study uses an engine-driven pump supplying hydraulic power to individual wheel pump/motors (P/M's) and/or an accumulator. Available P/M's have high efficiencies (e.g., 95%) at the ideal point of operation, but the efficiency falls off considerably at combinations of pressure, speed, and displacement that are significantly away from ideal. In order to maximize the fuel economy of the automobile, it is necessary to provide the proper combination of components, system design, and control policies that operate the wheel P/M's as close as possible to their maximum efficiency under all types of driving and braking conditions. This is accomplished by using four individual wheel P/M's with planetary gear reducers (with different displacements for the front and rear units) along with an innovative accumulator design that can provide hydraulic pressure at three alternative levels. The system control policy continuously optimizes the system pressure, the location and number of wheel P/M's being used, the operating parameters of the active P/M's, the charge level of the accumulator, and the operation of the engine/pump subsystem. The paper includes design details of a hydraulic chassis buck being built for validation and demonstration of the design concepts.