" the drag @ 85 km/h is about 72.24% of what it is @ 100 km/h. So drag decreases by 27.75%. Add to that losses from other sources like rolling resistance and you're saving a lot."
Both wind drag and rolling resistance burn gas, but the rolling resistance is linear with speed, so for backofenvelope estimate, it is somewhat Subtracted from the savings implied by the aero calculations. Also, the engine is usually less efficient at lower speeds. The scan gauge trumps all theories, though.
