Dar - the problem with the cheap tachs is you wind up getting to close to the prop while trying to avoid the effects (misreadings) caused by multipath. The TNC was a godsend at 99 bucks. There are much better tachs but they generally cost from $250 to a thousand and are available thru industrial catalogs.

The optical tach with an analog readout that many of the pylon guys use is real good but goes for about 200 bucks I think. SSAN would know.