A vehicle transmission retarder control apparatus is provided including an engine, a throttle, a brake, a transmission having an output member with an actual transmission output speed, a speed sensor operable for measuring the actual transmission output speed, a plurality of user-commandable input devices for selecting a desired transmission output speed, and a controller having an algorithm for controlling the amount of retarder request to provide a zero or constant rate of deceleration independently of weight and axle ratio. Additionally, a method is provided for controlling a transmission retarder, including measuring the actual transmission output speed, sensing a desired transmission output speed using a plurality of user-commandable input devices, communicating the actual and desired speeds to a controller, and commanding the retarder to apply a continuously variable opposing torque to the transmission based on the actual and desired output speeds to achieve a constant rate of deceleration.