Abstract: A device for providing a retarding force against the motion of a vehicle, which device does not intrude upon the engine, is not dependent upon the gear selected, and does not exhibit fade or skid characteristics. A torque limit clutch engages a ground engaging wheel of the vehicle with a first aerodynamic rotor. In one embodiment, the ratio of the speed of the aerodynamic rotor as compared to the speed of the ground engaging wheel is controlled by a serially connected variable-ratio gearbox. In another embodiment, the ratio is controlled by an epicyclic gear train, a second aerodynamic rotor and a pump.