The quest to improve the fuel economy of General Motors passenger cars has led to the investigation of an engine-flywheel hybrid powertrain at the GM Research Laboratories (GMR). An engine-flywheel system was designed for a compact car and its performance was predicted analytically. The system was estimated to achieve an improvement in EPA Composite fuel economy of 13% over a 1984 production compact car. This margin of improvement was judged insufficient to justify the complex drivetrain, and, therefore, a prototype system was not built. However, the current potential of engine-flywheel hybrids for GM passenger-car applications has been defined, and the technology areas requiring additional research attention have been identified.