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After testing, it appears that most bike computers lose a lot of accuracy over 1,000 RPM. I suspect this is probably because the reed switch used to sense the magnet simply can't operate that fast.
I had been planning on installing bike computers on both the mill and lathe, but the 1,000 RPM limitation doesn't make it worth my time since a lot of my cutting it done on aluminum which uses speeds often well over that.
As of now the plan is to continue using my non-contact laser tachometer and I'll mark graduations around the speed control knob. Crude, but it should be effective enough.