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

c

Engine, integral automatic transmission, driveline or trailer-axle retarders

It assumes that foundation 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.