Abstract: A drive mechanism for a bicycle comprising a pump unit driven by the rotation of the bicycle pedals, a hydraulic motor driven by the pump unit, a conduit system creating a closed hydraulic system for circulating hydraulic fluid between the pump unit and the hydraulic motor, and a pressure regulator for controlling the flow of hydraulic fluid from the pump unit to the hydraulic motor and thereby controlling the speed at which the bicycle is propelled. The pump unit has a first gear assembly having teeth with a face width w.sub.1 and a whole depth d.sub.1 and the hydraulic motor has a second gear assembly having teeth with a face width w.sub.2 and a whole depth d.sub.2. The teeth upon the respective gear assemblies mesh together to form inter-tooth pockets which facilitates hydraulic fluid transfer about the closed hydraulic system. The rotary motion relationship between the first and second gear assemblies of the present invention, as defined by their respective teeth, may vary from approximately 2.