And people who disagree with me are under the delusion that there is no vehicle attached to their vehicle's engine, when that engine is operating at it's most fuel-efficient RPM. Nobody would argue that an otto cycle engine does not have a most fuel-efficient RPM. Yet oddly, many people seem to think this has NO RELATION AT ALL to the fuel economy of vehicles equipped with these same engines. It is bizarre logic, in the extreme.
There is one component that provides kinetic energy to keep a "car" moving. (unless you are talking about a hybrid) That one component is most fuel-efficient at a certain RPM. In a vehicle with no transmission (engine connected directly to drive wheels, with perhaps a clutch to allow easier startup, ha ha), this would mean that the vehicle would be most fuel-efficient at a ground speed that equates to that particular engine RPM. Such a vehicle would not be very practical, though. So cars have multiple gears to allow the car to operate at a variety of different ground speed ranges efficiently.
Best fuel economy is achieved by running your engine at it's most fuel-efficient RPM in the highest gear available. This is because while the engine is using roughly the same amount of fuel as it would in a lower gear at the same RPM, the vehicle's ground speed is maximized, allowing the vehicle to maximize the number of "miles" travelled for each particular unit (ie gallon) of fuel consumed.
Now someone is going to scream "but what about wind resistance????" It's a red herring. At normal or slightly higher highway speeds, wind resistance is actually pretty low for the average car, but wind resistance does affect fuel economy somewhat. This doesn't change the fact that your car will be most fuel-efficient in it's highest gear at the otto cycle engine's most fuel-efficient RPM. BUT, without wind resistance, the vehicle would be even MORE fuel-efficient. That's why the wind resistance is a red erring. -Dave
