This transmission consists of : 1. A three-element torque convertor (impellor, turbine and reactor) forming a variable engine torque multiplication ratio ranging between 2,3 and 1. 2. A gearbox comprising mainly : - an gear train which makes it possible to obtain three forward gears and one reverse gear. - a hydraulic gear selector permitting automatic gear changing. 3. An oil pump, driven by the convertor impellor which feeds the convertor and the hydraulic gear selector as well as lubricating the gear train. Il. ADVANTAGES.