“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. Power is required to produce this vacuum, power that is not propelling the bike. This causes fuel mileage to be especially poor at low throttle, and the bigger the engine’s displacement the greater the effect.
2. Engine mechanical friction: The bigger an engine is, the larger its bearing surfaces must be. As we putt along the freeway at 75 mph on our literbike, we are using 20 to 30 hp but are burning extra fuel to shear bearing, valve train, and cylinder-wall oil films sized for 185 hp.
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. Why? Six speeds are the present norm for motorcycles, but autos are now being given eight, nine, or even 10 speeds. This gives them a first gear low enough for starting, a top gear tall enough to give economical low-rpm highway cruise, and ratio-to-ratio separations narrow enough for the engine’s torque range.
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. Well-streamlined autos approach a Cd of 0.3 (meaning drag is only 30 percent of that of a flat plate of equal frontal area), but motorcycle Cd is more like 0.6, similar to that of a bread van. The difference? Autos do have larger frontal area than bikes but can be fully enclosed in a smooth shape that pushes the air aside and then to some degree smoothly brings it back together to generate a minimal wake. Even bikes with fairings have frontal streamlining only and are too short to include a tapering tail to put the airflow back together. The result is a swirling, turbulent wake that carries away a lot of energy and increases fuel consumption.
For the moment, motorcycle fuel consumption is not regulated by the Corporate Average Fuel Economy (CAFE) standards applied to US autos and rightly so. There are far fewer motorcycles than there are cars, and there is value in the simplicity of the motorcycle (which is why, historically, motorcycles have provided low-cost transportation when people have needed it most, after major wars or other economic dislocations).
If motorcycle fuel consumption is regulated in the future, expect to see smaller engines (possibly boosted by supercharging or turbocharging), turning lower freeway revs thanks to having either more transmission speeds or some form of wide-range Continuously Variable Transmission (CVT).