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Originally Posted by bull27
I read somewhere on this forum that 4300-4500RPM range is the most fuel efficient, I might be completely off on this? In a 5 speed Boxster that would be 4th gear on the highway (approx 70-75MPH).
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That's where the engine makes the most torque, therefore it's where the engine's volumentric efficiency is highest. This means that it is most efficiently burning the fuel at this engine speed. Think of it as gallons of fuel per piston stroke.
When looking at the overall fuel efficiency of the vehicle, you need to look at the gallons of fuel per mile driven. As stated before, the lower the engine speed, the less fuel is being consumed per mile, even if it is slightly more fuel consumed per stroke, simply because there are alot fewer piston strokes per mile which more than makes up for the difference in volumetric efficency.