CDL General Knowledge TestDriving SafelyMedium
When driving a commercial vehicle in mountainous terrain, you encounter a long, steep downgrade. What is the primary reason for selecting a lower gear BEFORE starting down the grade?
- ATo allow the service brakes to cool down before they are needed.
- BTo maintain a higher speed for better momentum through the descent.
- CTo increase engine RPMs and utilize engine braking to control speed.
- DTo save fuel by allowing the engine to idle more freely.
Show answer & explanationAnswer & explanation
Correct answer: C. To increase engine RPMs and utilize engine braking to control speed.
Selecting a lower gear before starting a downgrade allows the engine to turn at a higher RPM, providing significant engine braking. This helps control the vehicle's speed without overheating the service brakes, which should be reserved for supplemental braking.
Why the other options are wrong
- A. Service brakes are used periodically, but engine braking is the primary speed control to prevent overheating.
- B. Maintaining a higher speed on a steep downgrade is extremely dangerous and leads to runaway vehicles.
- D. Engine braking uses fuel and is about control, not fuel savings.
Engine Braking on Downgrades
Using the vehicle's engine and transmission to slow down by selecting a lower gear, thereby reducing the reliance on and potential overheating of the service brakes, especially on long, steep descents.
- Crucial for mountainous driving.
- Prevents brake fade/failure.
- Lower gear = higher engine RPM.
- Should be done BEFORE starting the grade.
Memory trick: Downshift early, save your brakes, no worries!