I have a little one-pulse-per rev anemometer (wind speed meter). I'm trying to guesstimate the pulses per mph, and my answer is looking far too easy...
Approx dimension from the axle to the centre of the cups is 7cm. That gives a circumference at the cup centre of 44cm (in round numbers).
One mile is 1760 * 3 * 30.5cm = 161,040 cm
Dividing that by 44 = 161,040/44 = 3,660
And that looks far too close to 3,600 to be coincidence, and a 1.6% error is well inside my likely measurement error and estimation of pi... (hey, I was balanced on a fence to reach the damn thing to measure it. And it was raining.)
Which rather suggests that as we might expect the cups to move at wind speed, we can expect 3,600 pulses per hour per mile per hour. Which is one pulse per second per mile per hour. So if I average over, say, ten seconds and divide by ten, I'll get a direct reading in mph.
Sounds far too easy - any holes in my logic?