Re: Why I don't trust mechanical speedometers.
if the circumference is pi X wheel diameter, then that's 26" X 3.14= 81.64 inches, or 6.8 (approx) feet per wheel revolution
6.8 X 200 wheel revolutions is 13,600 feet, per minute
since there are 5, 280 feet in a mile, that equals just over 2.5 miles in a minute
which of course is over 30 miles an hour- so no, it appears that the revs scale at least does not match the speed scale
the accuracy of the speed scale could be determined on a pre-measured course at a constant rate of speed, 60 mph for one minute,
so say, 20 mph which would take 3 minutes to cover a mile.