I thought there was enough evidence that showed that the Firefly lost a few MPH due to both the extra weight and a different engine compared to the US versions of the M4???
Besides, the Firefly does not need any more speed. Everyone knows the British are not in a rush to do anything or go anywhere.
I researched for days to why the Firefly is so slow, and the only answer I could find was the Governor, which was quite common on British tanks which were typically much faster. I couldn't find exactly why they were slowed down so much, I assume its because of the Governor.
Now that book that shows 22.75 mph was under british specifications, from what I read it clearly said a never "exceed" speed on the Firefly was 25mph, I still have no clue other then I again assume its over the Governor or Suspension.