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When are exhaust brakes most efficient?

At high engine speed, while in low gears
Correct answer

DVSA explanation

An exhaust brake alters the engine's exhaust flow, using it to help control the vehicle's speed. It's most efficient when the engine is running at a high speed and the vehicle is in a low gear, such as when descending a long hill. Using the exhaust brake can relieve the service brakes, preventing them from overheating and fading.

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