Another Jack-shaft gear ratio with tire size calculator =
Rawge's Mini Chopper Information Site - Jackshafts
Tachometers?
GO TO ebay, or Amazon.com ... type in "tachometer hour meter" in their search. For about
$15 USD there are cheap induction models flooding the market now (12/2010). They need NO other wiring hook-up (or power supply) other than its own single wirewrap to your spark plug.
A single wire from tach wraps 3-4 times around the spark plug wire and senses the electrical pulse. It will work on many 2 stroke and 4 strokes (it's programmable per how many spark plug firings per revolution). Has an Hour Meter on it too.
Cheap, and Chinese, it will only last as long as it's lithium battery works (they say about 4 years, probably less)
They look like this EXAMPLE:
This is just an example I pulled from some Amazon sellers page
Amazon.com: Digital Hour Meter and Tachometer with Maintenance Settings: Automotive