At what engine condition is an engine retarder most efficient?

An engine retarder is most efficient at higher engine RPM and lower vehicle speed because, during this condition, the retarding effects of the engine are maximized. As the engine speed increases, the amount of vacuum generated in the engine increases as well, which enhances its ability to slow the vehicle down. The retarder works by using engine compression to create drag, and this drag is most effective when the engine is operating at higher RPMs where the engine is still able to generate significant resistance without stalling out.

At lower vehicle speeds, the increase in engine RPM provides a greater retarding force without requiring the driver to apply the brakes, which enhances control and reduces brake temperature. This is particularly beneficial in heavy vehicles where managing speed on downhill grades is essential for safety and mechanical durability. The combination of higher RPMs and lower speeds utilizes the engine's mechanical capabilities to aid in braking rather than relying solely on the conventional braking system. The other options, while having some merit, do not combine these critical factors in a way that maximizes the efficiency of the engine retarder.