G'day

If you are the USA and you point the tachometer at a light which is driven from the mains (fluro or incandescent) it should read 3600 rpm. You have 60 Hertz mains. Here in Oz we have 50 Hertz mains which means my tacho reads 3000 when set for a two bladed prop and I point it at a light bulb or fluro tube. A simple test for tachos.

Each cycle of the mains produces two pulses (as does your two bladed prop per revolution).