Fuel management system for operation of a spark ignition gasoline engine. The system includes a gasoline engine powering the vehicle and a source of gasoline for introduction into the engine. A source of an anti-knock fuel such as ethanol is provided. An injector directly injects the anti-knock fuel into a cylinder of the engine and the control system shuts down the engine by stopping gasoline and anti-knock agent flow into the engine during vehicle deceleration and idling and restarts the engine upon driver demand. Direct ethanol injection and engine shutdown results in efficiencies similar to those of full hybrid vehicles.