Final answer:
Forcing an automatic transmission into a lower gear while driving on a downgrade can reduce the speed of the vehicle without excessive braking and without affecting fuel efficiency or engine RPM.
Step-by-step explanation:
When an automatic transmission is forced into a lower gear while driving on a downgrade, the speed of the vehicle may be reduced without applying the brakes excessively. The lower gear allows the engine to slow down the vehicle by using engine braking, which can help control the speed and prevent overheating of the brakes.
Other aspects that are not affected on a downgrade include the fuel efficiency of the vehicle and the RPM (revolutions per minute) of the engine, which remain relatively constant as long as the throttle is not manipulated.