[Christ]

The engine doesn't spin any slower when you reduce load. An engine, at any point where it is not accelerating or decelerating, is idle. The way we tend to use the term idle is quite a misnomer. Idle means at rest, or not laboring. When you load the engine, once it stabilizes, it's effectively at idle again, even though it's at a higher speed than normal "idle".

Anyway, the point is that an engine only produces exactly enough power to satisfy it's current operating status. If you demand 30 mph, the engine will produce enough power to maintain 30 mph in addition to overcoming static and parasitic loads. Just because the RPM that coincides with 30mph happens to be capable of producing twice the required power, doesn't mean that it is producing that much.