During city traffic or heavily congested roads, a vehicle can consume a substantial amount of fuel idling when the vehicle is stopped. Due to regulation enforcement, auto manufacturers are developing systems to increase the mileage and reduce emissions. Turning off the engine at traffic lights and regenerative braking systems are simple ways to reduce emissions and fuel consumption. In order to develop strong manufacturer and consumer interest, this type of operation needs to be automated such that the stop/start functionality requires no driver interaction and takes place without the intervention of the vehicle operator. Valeo Electrical Systems has developed such a system that replaces the OEM engine alternator with a starter/alternator driven by a standard multi-ribbed V belt. To avoid a break and dual voltage network, this system is based on a 12V electrical system using an Enhanced Power Supply. In 2004, this system was designed for and integrated into a 2003 GMC Envoy and is capable of starting the vehicle's engine very quickly and quietly. In order to obtain smooth, transparent start/stop operation on the road, a substantial amount of integration work with the factory PCM, engine, and transmission was accomplished. During operation, the Start/Stop system can start and stop the vehicle very quickly and is able to produce acceleration from vehicle rest that feels identical to a production factory vehicle. Baseline vehicle fuel economy was measured for the OEM configuration and after installation of the start/stop system using standard chassis dynamometer fuel economy test procedures. This testing demonstrated 5.3% improvement in the city cycle and found approximately a 4.0% improvement in the highway drive cycle due to regenerative braking. Future development of the start/stop system was done using a forward looking vehicle simulation model to determine the fuel economy impact of modifying this system to operate as a mild hybrid electric system. Using the vehicle simulation software (RAPTOR), it was determined that hybridization of the start/stop system could obtain a 10% improvement in the city cycle and a 5% improvement in the highway drive cycle, based on the applied system architecture and design. This type of system can be easily integrated into a production vehicle and could provide significant fuel economy improvements with minimal cost or affect on vehicle performance.