Dave C.
The implicit buttumption in that sort of "rule of thumb" is that you NEED all the power that the engine can produce. Its a good rule for, say, an industrial generator engine or irrigation well engine that is expected to run most of its life at maximum load Cars cruising down the highway NEVER need all the power that the engine can produce, so they play by a different set of rules.
Cars have engines that are capable of producing several hundred horsepower in order to achieve good acceleration, but when cruising steady-state they require a few tens of horsepower at most. So how do you run a 300-horsepower engine at an output of 35 horsepower and do it most efficiently? It turns out that the best way is to "lug" the engine. IOW, run it so that its way down on the lower RPM side of its torque curve, but do so with the throttle held relatively wide open at that low RPM. that's why overdrive works so well. Lugging the engine does a few things simultaneously:
1) it raises the starting pressure in the cylinders by decreasing the efficient combustion
2) it allows the spark timing to be retarded. Spark "advance" is really only needed because when you operate an engine against very high manifold vacuum (throttle mostly closed) you are LOWERING the starting cylinder pressure so much that the burn rate is reduced, so that in turn you need to start the burn very early in order to finish it before the exhaust valve opens. The downside to spark advance is that a portion of the combustion cycle is working against the movement of the piston. If you lug the engine with the throttle open, the pressure is high so the burn rate is high, and you don't need much spark advance, and therefore less of the combustion cycle is spent working against the crankshaft.
The proof is in the pudding. If you look at that (otherwise useless :-) ) tach in any modern car, you'll see that when cruising in O-D at 70 mph, the engine is only turning 2000 RPM or even less. And this is usually with engines that have redlines of 5000-7000 RPM and torque peaks up at 3000-4000 RPM. By any rule of thumb relating to their MAXUMUM power, the engines are being operated way, way off optimum. But for the NEEDED power, they're right in the sweet spot.
