With regard to suggestions in the operator's manual, I would also keep in mind Mike K's point:
Henry's original 40 hp engine did not produce sufficient torque to do compressive damage to the original diameter, thin babbitt. Adding torque (by upping the Hp. with mods) or lowering the bearing compressive limit (shims, journal regrinds, changing the contact profile with excessive clearance) will leave you with an engine that is susceptible to damaging itself.
The original recommendations in the original operators manual were probably "Safe" when the cars were new, the engines were at factory tolerances, and if you did trash the bearings, replacement rods or a quality babbit job were a lot cheaper and easier to find than they are nowadays.
10 MPH sounds a bit slow for high-gear.
The speedo in my '28 reads about 3-5 mph "fast" at 50 mph, so I imagine we should make some allowances for speedometer error between individual cars.
On level ground, I usually make the 1-2 shift around 10 MPH, and the 2-3 shift around 20 MPH.
When the car speed drops below 20 mph ( 25 when pulling a hill ), I will down shift (Or at least think about it
). I almost never drop below 15 MPH in high and try to accelerate from there.
The heavy flywheel and low compression of a stock A are somewhat deceptive, in that they will allow the operator to lug the engine way down, well below the point where a more modern engine would buck and probably stall... that doesn't mean it's a good idea to do it frequently...
Your results may vary...
), I'll down-shift.