A hydraulic drive system incorporating a hydraulic pump for three wheeled vehicles such as “piggy back” forklifts in which there are at least two separate but linked drives forming one drive unit for each of the wheels and each drive has a separate input and output. Valving is provided to provide both parallel and series operation of the system for optimum torque, speed and anti skid operation. In one preferred way of carrying out the invention a single drive unit feeds one of the drives of each of the other drives. The hydraulic pump feeds the single drive unit and one of the drives of the other transverse drive units. By-pass diverter valves are fitted across the drives of the transverse drive units.