Robert,


I would second Tony's recommendation.
I have the Hobbico tachometer and it works pretty well even on cloudy days.

You sometimes have to change viewing angles to get a consistent reading, but experience will easily teach you when it is not showing what it should.
The only thing that really affects it... don't take readings in close vicinity to extreme voltages (high tension wires, F.E.), or your 10,000 RPM would appear to be 31,000...

For $20 you can't go wrong.