Originally Posted by Dragin
Any engine is at its most efficient, when its RPM is HIGH, and
the load on it is LOW. When you are in a high gear, the load on the
engine is greater, and a slight up hill can easily reduce the RPM,
meaning that there is a greater load on the engine. By shifting down a
gear at that moment, you will effectively reduce the load on the
engine, and the RPM is able to come backup. Yet you have not pressed
the accelerator down any further, thus your fuel (energy) consumption
is still constant.
They're talking about effieciency in relation to power, not to fuel consumption. If engines were most fuel efficient at higher RPM, we'd have no reason for transmissions. At higher RPM, even though you have not introduced any additional air, you're introduced additional RPMs. In an 4 stroke engine, you always fire on the ignition stroke and always intake on the intake stroke. You need fuel to do both, and more RPMs = more intakes and ignitions = more fuel delivered. Test it yourself. Drive to work in 1st gear one day and then drive to work the next day in high gear. See which one uses more gas.
Also, higher RPM = higher vacuum. In a carbureted engine, that's going to suck more fuel, probably through the lager main jets vs. through the smaller, more fuel efficient pilot jets. In a fuel injection car, your MAF is going to sense more air or you MAP is going to sense less manifold pressure, and provide the car with more gasoline to compensate for the RPM.
Engines generate more power at higher RPMs, but are certainly not most fuel efficient at high R. My 307 cubic inch Cadillac motor sits at ~1000 RPM all day with its low gearing and gets the same mileage as a V6 and some I4 motors that run higher RPMs.