Abstract: An automatic transmission device for vehicles having an automatic transmission ratio variator formed by two expandable pulleys interconnected by a trapezoidal belt, hydraulic fluid under pressure being fed to the pulleys to control the distance between two half-pulleys forming each pulley under control of a valve subjected on two opposed faces to the vacuum in the engine induction manifold and to a fluid pressure dependent upon the speed of the driving pulley respectively. The face of the control valve which is subjected to the said fluid pressure is divided in effect into two separate faces one of which is connected directly to a pressure transducer response to the engine speed and the other of which is connected to the said transducer through a selector valve which may be a solenoid operated in dependence upon the accelerator pedal position.