If the tach has TWO wires as shown in mmisterbungl's photo it is probably one of the better quality ones (e.g. "Tiny Tach"). If , on the other hand, there is only ONE wire, it is likely one of the cheap Chinese knock offs, (typically sell on Ebay for about $ 20.00) that are REALLY crappy in my opinion. I bought one early last year and found it to be extremely inaccurate and displayed wide RPM fluctuations even with the throttle held in a stationary position!::doah: I would NEVER trust it for use in any type of engine analysis or (especially) governor calibration!
ut: When I brought it to the attention of the seller they issued me a full refund AND said "just go ahead and keep the tach." While I appreciated their customer service, that seemed too easy to me almost as if they were already aware the tach was no good.:shrug: