I don't think David has an area on his web site to calculate the needed "tooth count" of the driven gear (speedo gear), which varies a lot. The "drive gear" on the output shaft also varies a lot in tooth count and size. It's complicated...
This MIGHT help:
http://www.sccoa.com/faq/speedgr.html
Once you get "in the ball park":
The number of teeth on your speedo gear is?
Your true speed compared to your speedometer reading is?
True speed is 50, speedo indicates 70?
Then the speedo gear is spinning to fast, gotta slow it down by adding more teeth. If you have 16 teeth, the maybe a 20 tooth gear would work? NOW we need a "math wizard"!
