The reason engine RPM is not directly proportional to vehicle speed, even when gears are not changed, is because there is always some resistance to the engine’s rotational speed. This resistance is due to things like friction, wind resistance, and the weight of the engine’s parts. When the engine is running at a higher speed, there is more resistance to its rotation, and thus the engine will slow down more quickly. When the engine is running at a lower speed, there is less resistance to its rotation, and thus the engine will slow down more slowly.
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