I use the [link=http://hangar-9.com/Products/Default.aspx?ProdID=HAN111]Hanger 9 Tach/ESV[/link]. I like it because it has a tach in it that will read 2,3, and 4 bladed props. But it also has an Extended Scale Voltmeter on it which is great for checking your batteries with. It's two different tools in one.

As an aside piece of FYI info, here's a good way to check the accuracy of your tachometer. Point it towards a florescant light source. On a two bladed setting it should read 36/360/3600 RPM. This will vary as to what exactly is read from model to model, but it should still be a factor of 60 of some sort. If it reads a value other than a factor of 60 then more than likely your battery is weak. This applies to countries that use 60 hertz electric setups, US and Canada. A lot of European countries will have 50 hertz electric and the readings will be a factor of 50.

Ken