A vehicle drivetrain has a first driver, a first clutch to selectively couple the first driver with an engine, an energy storage device coupled to the first driver, a second driver coupled to the energy storage device and connected to a driveshaft, and a second clutch that selectively couples the first and second drivers. In one embodiment, the first clutch is opened and the second clutch is closed during a vehicle operating condition requiring high torque, the first clutch is closed and the second clutch is opened during a transient vehicle operating condition, and the first clutch and the second clutch are closed during a steady-state vehicle operating condition. Additional clutches may be included to reduce energy losses or provide multiple mechanical ratios. The drivers themselves may be pumps, motors, generators, combined pump/motors, or combined motor/generators, making the invention suitable for both hydraulic implementation and electric implementation.