Abstract: Gear transmission system particularly for a motor vehicle. It comprises a shaft assembly made up of an outer driving shaft and an inner driven shaft, both coaxial, with the driven shaft extending out of the driving shaft at both ends. The driven shaft is formed with an internal gear circumscribing the shaft, a sun gear integral with the driving shaft being located within the internal gear and a planetary gear support radially extends from the driving shaft, being mounted thereon for free rotation and slidable axial displacement therealong. The support has rotary planetary gears meshing respectively with the sun gear and with the internal gear. A transmission member, integral with the driving shaft, is disposed in radial facing relation with the gear support and provided, along with the gear support, with cooperating gear teeth suitable to mesh together for forward driving of the vehicle.