Leo, I believe those speedometers use a magnetic sensor, not a reed switch.
The one I have is adjustable for wheel size, but does not go anywhere small
enough. They are cheap, though.

100 foot per minute is 1.1 mph. Playing around with it might get you to the
point where the readout is x100 rpm.

Graingers has digital rpm meters.

I have an rpm attachment for my DVM.

Dan


"Leo Lichtman" wrote in message
...

DJ Delorie wrote: I've measured my lathe's actual RPM using a magnet, a
coil, and a digital oscilloscope (clip)
^^^^^^^^^^
I recall reading somewhere about using a bicycle speed meter (cyclometer.)
You mount a magnet somewhere, like on the handwheel, and a little reed
switch next to it. Cyclometers have a setting for wheel size, which
should
allow you to make the thing direct reading. It won't feed your competer,
though.