The invention relates to a compound transmission for a vehicle such as an agricultural, forestry or publics works tractor, combining a gearbox transmission and a transmission of the hydraulic type. This transmission includes a clutch of the dry disc type, the front plate of which is driven by the heat engine of the vehicle, and the back plate of which drives a gearbox including a direction-of-operation reverser the gears of which interact directly with the primary shaft of the gearbox, a gear ratio selector making it possible to select a step down gear ratio between the layshaft and the secondary shaft of the gearbox, and a range selector the gears of which interact directly with the secondary shaft of the gearbox. It furthermore includes a hydrostatic unit composed of a variable-output hydraulic pump driven by the heat engine of the vehicle, and of a hydraulic motor connected to the hydraulic pump by a closed hydraulic circuit, the hydraulic motor driving the layshaft of the gearbox through the use of a clutch and of a transmission assembly connecting this clutch to the layshaft, the adjustment of the fluid pressure in the hydraulic circuit being carried out by varying the cylinder capacity of the pump using an operating member, the variation in the fluid pressure in the hydraulic circuit giving rise to the variation in the rotational speed of the hydraulic motor, and a synchronising device making it possible to synchronise the gears of the direction-of-operation reverser. <IMAGE>
