You may be correct, but IIRC, the Model A is about 2200 RPM at roughly 50 mph, which would (likely) put the top of it's efficiency curve about 1600 or 40 mph?
Think of driving the car at about 43 mph as equivalent to synchronous speed?
I don't have the ratios at my fingertips here so would appreciate a check.
Still, synchronous service is a bit tough on an engine. It can be CONSTANT loading at full output - loading which almost never happens except in acceleration in an automobile. Most engines in modern automobiles loaf along at less than 1/4 their maximum output at any given speed.
Belts work. In fact, before settling on the 1800 ST generator head I briefly considered going with a serpentine belt to the Harbor Freight 10kw machine (which is actually marketing upgraded from 7.2 kw in advertising hubris.) This much as the Listeroid people have done. Their engines (no longer available except as do-it-yourself parts thanks to your EPA) come "pre-grooved" to accept a serpentine belt and rotate at up to 850 rpm. (depending on horsepower.)
Whatever the motive power direct connected HAS to be more reliable than belt connected though. As was found out in central station practice early in the 20th century.
Joe K