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But it's more than just the efficiency of the engine at play. If the engine were "too powerful" - and you had the gears for it - you may have to drive 100 mph+ to be at the peak efficiency RPM. But the aerodynamic drag in that situation would wipe out the small efficiency gain of the engine. And running in a lower gear just to keep the revs up to the "most efficient RPM" would cause worse MPG, too.
Also, if that engine is "too powerful", pulse & glide would become less effective too, because it would accelerate you too fast, making that most efficient RPM range just a momentarty "blip" on your trip profile.
I came to this conclusion when trying to decide what speed to climb hills at in my Jimmy. According to the BSFC charts for my engine, it would be 2800 RPM at around 75% to 80% engine load. Well, when I tried to go up the hill under those conditions (via my ScanGauge), it downshifted like mad (a disadvantage of an automatic), and then accelerated right past the speed limit in a lower gear. - and my MPG sucked big-time. Now, if it wasn't an automatic, maybe I could have found a speed in top gear that would allow me to work the engine that hard at that RPM . . . . . the penalty of the automatic, I guess.
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