Using Frank's figure of 1100ft/min +/- 100, my engine would be most efficient at 1700rpm +/- 150, or 55mph +/- 5mph.
Something I wonder about, is how ignition timing plays into this.
Here is one of my ignition timing tables, which I built based on maintaining a roughly fixed peak cylinder pressure:
When spark ignited, a flame front spreads from the spark plug through the combustion chamber, and it's ideal to have peak cylinder pressure occur when the piston rod has the best mechanical angle to spin the crankshaft. Flame speed is pretty constant for a given fuel and air mixture, so as the piston moves faster (higher RPM), you need to start combustion a few degrees of rotation earlier to keep peak pressure at the same crank angle.
At higher engine speeds, there's a larger percent of of high pressure before the piston even reaches the top (trying to spin the engine backward) and also too late in combustion, where the angle isn't very good. At least, to a certain point. My understanding is that, although the piston moves faster as RPM increases, after a certain point one does not need to ignite it any earlier, and indeed my ignition timing tables suggest this is true. I don't have a firm grasp yet on why this is so.