justjed:
... But with 8 of them around the perimeter of a 5" diameter drum, will the flux density at the midpoint between 2 of them be low enough for it to turn off?
My recommendation is to use a Hall sensor that turns on when it sees one polarity of the magnetic field, and off when it sees the other. Such as the US1881. It makes for a more reliable system, and doesn't cause false tacho pulses if the wheel wobbles back and forth slightly when the vehicle is stationary. Of course, this means you need twice as many magnets (with alternating polarities) to get the same frequency. But you can mitigate this by counting edges rather than pulses, thereby doubling the frequency.