Abstract: An overdrive assembly is adapted to be retrofitted into an automobile between the engine and the automatic transmission. The overdrive can be retrofitted to most domestic cars and light trucks equipped with automatic transmissions. The overdrive can be retrofitted without modifying the automobile chassis and without introducing any additional controls into the automobile.The overdrive assembly includes a housing rotatable at the speed of the engine. This is accomplished by coupling the housing of the overdrive assembly to the engine. A gear assembly converts the rotation of the housing to a rotation of a gear in the assembly at a speed greater than the speed of the housing. The gear assembly may include a stationary sun gear, a planetary gear rotatable with the housing and a ring gear rotatable at a speed greater than the speed of the housing to provide the overdrive. The ring gear drives an impeller. A turbine is hydraulically coupled to the impeller to rotate at the speed of the impeller.