Abstract: A gearbox to drive six accessory units from a vehicle prime mover or alternatively from an auxiliary engine. A continuously variable ratio unit in the drive line from the prime mover provides a constant speed at a first input to the power transmission gears over the operating range of the prime mover.The auxiliary engine operates at a constant speed, driving a second input to the gearbox and by-passing the variable ratio unit to drive the accessories at the same constant speed as that provided by the prime mover.Over-running clutches permit the change-over to drive from one power unit to the other to take place while both are in operation.