The procedure covers the estimation of the total retardation capability available to a specific vehicle from: a) natural retardation (rolling resistance, aerodynamic drag, etc); b) engine drag; and c) engine, integral automatic transmission, driveline or trailer-axle retarders. It assumes that foundations brakes are not used for maintaining speed on long mountain descents. Retardation is rated in terms of the maximum grades on which stable control speeds can be maintained for each gear over the range of highway speeds appropriate to that gear. For each gear, the calculation procedure determines maximum grades for at least four values of control speed ranging from the vehicle velocity corresponding to full load governed engine rpm, to the vehicle velocity corresponding to the engine rpm at minimum (idle) speed. in addition, the calculation procedure provides information on the total retarding power available for each gear.

This procedure provides a uniform method for calculating and reporting the steady state control speed maintainable by either engine, integral automatic transmission, driveline, or trailer-axle retarders employed by heavy trucks and buses operating on downgrades. It provides a practical method of predicting retarder performance in a specified vehicle installation using data describing retarder performance characteristics and other vehicle design variables. It is intended to assist in the proper selection of a retarder by matching stated retarder performance capabilities with particular vehicle functioning requirements.