A hybrid drive system for vehicles. The system includes the connection of an engine and a motor, one as a combustion engine (1) and the other as a pneumatic motor (2), with this connection being made through a mechanical coupling (3) and, at once, through a pneumatic connection between the exhaust (8) of the pneumatic motor (2) and the input (9) of the combustion engine (1), including a control unit (19) which controls both the coupling (3) and the engine (1) and motor (2) and supplementary equipment associated to the latter. The pneumatic and mechanical coupling between both engine (1) and motor (2) provides comburent overfeeding to the combustion engine (1) and, at the same time, a sum of the drives provided by both engine and motor, with this sum of drives being applied to the pertinent clutch (6) of the system, in order to achieve a reduction in the consumption of the latter and greater power.