Abstract: A motor vehicle engine is arranged to drive the vehicle wheels through a mechanical transmission, an hydraulic system including a variable-output, reversible, hydraulic pump, a variable-output hydraulic motor usable reversibly as a pump and a two-speed gear box. The outlet and inlet of the pump are reversibly connected by delivery and return conduits respectively, through a distributor, to the inlet and outlet of the motor and an hydraulic accumulator is connected through a closable valve to the delivery conduit, the operation of the system being controlled by a microcomputer so that when the vehicle decelerates at a rate greater than that due to natural resistance, kinetic energy which is normally lost in the engine is stored in the accumulator and can subsequently be reused to drive the vehicle wheels, the engine being cut out or maintained at minimum power.