A power train for a motor vehicle includes a combustion engine, a clutch or other torque-coupling device, a transmission, and an electro-mechanical energy converter that is operable at least as a motor and as a generator. The electro-mechanical energy converter is coupled to the output shaft of the combustion engine through a torque transfer device with at least two rpm ratios that automatically set themselves according to whether the vehicle is operating in a start-up mode or in a driving mode.