Abstract: This invention concerns a motorizing device for a vehicle, of the trailer category which is attached to a tractor equipped with a main engine organ. In particular, placed on said vehicle, the device includes: an auxiliary thermal engine (20), a plurality of hydraulic engines (50) respectively associated to each drive wheel of the trailer, an equal number of pumps (31) respectively connected to hydraulic engines (50) and associated to the auxiliary thermal engine. The device includes in addition a regulating element (100) which acts either on the hydraulic engines (50), or on the pumps (31), which is sensitive to the rotating speed of the main engine organ of the tractor.