Chalenge, granted you have provisionally accepted the 22.25 mph, but your implication that you can calculate a tank's speed from original data has not yet been backed up by evidence. It is not reasonable to expect people to take your word for your level of expertise in this, especially when it contradicts published literature. Surely you can see this?
Finally, I have never driven a WWII tank. Still, the tank literature and miniatures rules which I have seen over the last 40-years-plus of wargaming tends to characterize a road speed and a cross country speed, with the cross-country tending to be about half the road.
It's really simple, Deacon, and whether I can calculate a tank's speed has nothing to do with it. Just look at the facts.
The M4A3 has the GAA engine in it which produces 500 hp at 2600 rpm, which translates to 26 mph. The M4A3 weights 59,560 lbs. (this is all in the manual TM9-759 that I referenced previously) and on page six of the technical manual for that tank it clearly states that the M4A3 can manage 26mph on road, and 4-26mph cross-country for various terrains. I also told you previously that there are countryside surfaces that tanks will run at road speed upon, yet you adhere to some other thinking. In AH those other terrains that slow tanks down are sloped terrains. Your experience in other games does not change that. The treads on these tanks are of type rubber-block, or steel. Perhaps that would make a difference, I don't know.
As to the power of the Sherman VC Firefly? Like I said before, 470 hp for the Chrysler engine has been reported, yet the same engine on the M4A4 is reported at 470 hp, but only 425 hp for the Firefly? If, it has less horsepower and more weight, then it simply cannot go the same speed. You should agree to that! Now simply break it down into available horsepower per ton terms and you can see there's a problem. If the added weight of the Firefly mod is enough to slow the tank to 22.25 mph, then taking away 50 hp is going to slow it down even more. HTC may have found evidence that 425 hp is the correct figure, in which case their speed limitations may also be justified.
EDIT: corrected the name of the M4A3 which is the tank of the TM9-759.