Apparatus for installation in new motor vehicles or for retrofitting into existing motor vehicles, including an electric motor coupled to a drive shaft and a controller for the motor. The existing vehicle battery and/or an auxiliary storage battery can be used to power the motor to drive the vehicle during certain driving conditions, resulting in greatly increased fuel economy, without interfering with the normal performance of the vehicle. A kit of parts may be provided for simple and economical installation on existing vehicles. A motor vehicle so equipped.