Once again: the most fuel-efficient point in an engine's operating range is (my apologies to all for strong emphasis) a COMBINATION OF RPM AND LOAD. You CANNOT determine, or even estimate, an engine's fuel efficiency solely from rpm!
An engine's efficiency map is THREE-DIMENSIONAL. It is a MAP of efficiency vs. rpm AND LOAD. It is NOT a 2-D curve of efficiency vs. rpm!
An engine's efficiency can be measured at the rpm corresponding to the best-efficiency rpm-load combination, but the engine's efficiency will change TREMENDOUSLY with load. At that rpm, the engine's efficiency at 1% of full load will be a tiny fraction of the engine's peak efficiency (at ~80% of full load).
That's just one ESSENTIAL concept that you don't get.
Here's another:
In fact, aerodynamic drag (or "wind resistance") is an EXTREMELY important determinant of highway fuel economy.
In fact, the relatively rapid rise of aero drag power at highway speed is the reason that INCREASING speed causes a DECREASE in fuel economy (fewer mpg) even though engine efficiency (bhp-fuel rate) is INCREASING SIMULTANEOUSLY.
You continue to think you understand the interrelationship between vehicle speed, engine rpm, and fuel usage, but you you're not even close.
Here's a clue: if you can explain or predict common, well-documented speed vs. mpg relationships, then you might have a useful rule of thumb.
So (once again): what fuel economy should a 6-speed Corvette achieve at ~125 mph in 6th? According to your rule, that should be a Corvette's best mpg cruising speed. In fact (and purely by coincidence), the engine will be very near it's most efficient operating point. EPA ratings for 6-speed Corvettes are ~18-26 mpg. -- Chuck Tomlinson
