Gryphon

I've only used a tach a couple of times and was never able to get a good stable reading.

But I do remember reading about checking their calibration.

Since the inside lights run off of 60HZ means 60cycles/sec, and you have 60 seconds in a minute.

Just holding the tach indoors without pointing it at any motors, it should read 3600 RPM.

Make sure you take all your readings outdoor.

I'm no expert but I hope the above helps make sure that your tach is All right.

Good luck.