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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; although they hydrostatic types have been used in main battle tanks. They are unlikely to be adapted for marine use.