Abstract: An hydraulic drive mechanism for tricycles and the like comprises a variable volume, positive displacement hydraulic pump and a hydraulic motor. The pump is dual chambered with each chamber operating a separate hydraulic system for separate drive wheels. Each chamber has three curved pivoting vanes around a central crankshaft. Each chamber also has two reservoirs which momentarily store hydraulic fluid. This temporary storage of fluid changes the volume output of the pump. Changes in pump volume output change drive ratios. The amount of fluid stored is controlled by the operator. Consequently, the drive mechanism changes drive ratios hydraulically. Also, the pivoting vanes in each chamber can be forced down by back pressure. This back pressure is generated by the hydraulic motor on each rear wheel. During cornering the outer radius wheel will rotate faster than the inner wheel and consequently will drive the hydraulic motor and generate back pressure and that pushes down the vanes.