Tim Wescott writes:

Problem statement:

Mount a small (nominally 3.2 x 1.6mm) NdFeB magnet in an aluminum model
airplane spinner backplate (for a tachometer).

The thing needs to work with a model airplane engine, so it's going to be
a high vibration environment.

A tach for model engine speed? If your sensor is a reed switch, it
almost certainly won't open and close fast enough. (As another poster
has observed already.)

I did this with the exercise wheel for my pet deer mouse that works
like a charm but flat out he's only running 4 mph max. (You do the
conversion to RPM of a 6" wheel. :-)

Take apart an old (computer) mouse and get the LED and sensor. Should
be far more responsive than a reed switch. (But I'm only guessing.
Electronics weenies to the bridge, please. Is there such a thing as a
high-speed, solid state magnetic switch?)

--
Mike Spencer Nova Scotia, Canada