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[https://en.wikipedia.org/wiki/Continuously_variable_transmission#Hydrostatic_CVTs Continuously Variable Transmissions ] (CVT) are mentioned here only for completeness. A CVT has a nearly infinite range of gear ratios. Like a controllable-pitch propeller, a CVT allows the engine to operate at its optimum rpm regardless of the speed. Until recently, CVTs were too expensive and unreliable for use in cars. They are unlikely to be adapted for marine use; although they have been used in main battle tanks.