I bought one of those mini tachs/ hour meters for my riding mower, and it does what I needed. It works fine for any engine that uses a governor, and is therefore supposed to operate at a nearly constant rpm. However, it is not suitable for any engine that is increasing or decreasing in speed rapidly, since the tach readout will not keep up with the rpm change. So, for instance, the tach would be useless for a stick shift automobile where you might use the tach for your shift points. When the rpm is changing rapidly, the lag between the actual engine rpm and the tach readout can be as long as 3 or 4 seconds. The nominal sampling frequency for the tach is 1 second, but there's a lot more going on here , and a stabilized reading takes much longer. I'm also a bit irritated that those mini tachs have an internal, non-replaceable battery that lasts for about 5 years, and then you have to buy a new tach. The whole circuit is potted in epoxy, so replacing the battery is mighty difficult. The tach I bought from Northern is adjustable for 2 and 4 cycle engines, and with a plug firing every revolution or every other revolution. Since the tach signal is from the spark plug wire, you'll need to get that right. ( Some 4 cycle engines fire the plug on every revolution, like a 2 cycle, but the firing on the exhaust cycle is not necessary.) Not all of those mini tachs are as adjustable, so be careful which one you buy.
