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 premeasured 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.
