Join Date: Jan 2007
Location: Sacramento, CA USA 96814
It's a little more complicated than that.
1. Drag losses increase with speed, exponentially. The slower you go, the less drag you have, and less energy is required to go the same distance. That is of course, obvious.
2. Engines are most efficient not at lower RPM's, but rather at specific RPM's, AND loads. If you run an engine at a higher, or lower load, or at a higher, or lower RPM than that which results in the highest BSFC (brake specific fuel consumption) then you are sacrificing efficiency. For most engines that means a nearly fully loaded engine running at somewhere around its peak torque curve.
3. Obviously, a fully loaded sportbike engine running at its peak torque curve would be driving that bike at a very high speed, resulting in very inefficient running conditions, even though the engine itself may be operating at its optimal BSFC. Conversely, cruising around in third gear, slightly off idle going 25 mph results in another inefficient running condition, since engine efficiency is very low, even though minimal power is required to propel the bike.
4. So there is a tradeoff, and an optimal speed for the most efficient running conditions. For most cars and light trucks, that speed is by design somewhere around 50-60 mph. This translates in to running the vehicle in the highest gear without lugging the engine, and then a slight amount. Again, that's still an inefficient running condition with respect to the engine, but the optimal tradeoff between engine efficiency and vehicle drag.
5. I notice myself, that there is surprisingly little difference in mileage on my bike, whether I cruise at 60 mph, or 90 mph. Obviously, there is a whole lot more incremental drag at 90 mph, but on the other hand the engine is running much closer to peak efficiency, which provides some degree of offset.
Last edited by Mister Tee; 10-10-2007 at 07:12 AM.