I wouldn't bother with an aftermarket tach if I were you. The factory available tach/clock combo are simple to install and work flawlessly with no calibration.
The ScanGauge is an excellent product for a lot of reasons, but a digital tach is... well... clumsy for everyday use. An analog one is far easier for data that's constantly changing like that especially when you don't really care if you're at 5000rpm vs 4986rpm or whatever.
I installed a smart tach/clock combo just the other day. I first was using the genuine smart installation instructions available on their electronic WIS but it really isn't very well written. A much better guide is found on
Kane's site over at evilution.
Get a ScanGauge though ;) Good for so many other things.
- Steven