“Why does my sportbike get such poor fuel mileage?” is a question we are asked all the time. Sometimes the questioner adds that the family four-door sedan gets better mileage than the motorcycle. Several factors combine to produce this result, as follows:
1. Pumping loss: As we throttle an engine back to low power, intake vacuum—produced as the intake strokes of the pistons try to pull air past throttle plates that are nearly closed—increases.
2. Engine mechanical friction: The bigger an engine is, the larger its bearing surfaces must be.
3. Excessive rpm: Engine mechanical friction increases roughly as the square of rpm. While economy-car engines spin at 2,200 rpm at freeway speeds, a sportbike engine is turning at least twice this and in some cases at as much as 8,000 rpm.
4. High aerodynamic drag: The so-called drag coefficient (Cd) compares the drag of a given shape to that of a flat plate of equal frontal area, perpendicular to the flow.
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