That sucks!! bet you have my old one..
The speedo rules, first time I felt I had an accurate speed reading - super accurate.
You need three bits of info:
1. Number of teeth on the speedo cog in the hub (bit in the hub the cable plugs into)
2. Number of teeth on the axle/hub drive
3. Circumference of your tire.
For the tire, get a bit of chalk and mark a line on your tire, then mark the ground under the mark to line up a starting point. Move the scoot forward till the tire mark rotates one full time and arrives back at the bottom of the tire and mark the lowest point on the ground. Now you have two marks on the ground, get a tap measure and measure the distance between the two marks you made on the ground - that will be the circumference (distance your tire travels every rotation).
The speedo needs the ratio between 1 and 2, divide the two numbers then find the corresponding ratio in the list options on the speedo. Then add the circumference.
It works by counting the revelations on the speedo cable then applying the rotation of the two gears (hub and speedo), very clever, especially not relying on magnetized resistance nonsense of the old units.
Good luck with the unit -
Be really careful of the spark plug rings, the get torn up very easily, good idea to get some spares.
1966 VBC - Motor: 2002 LML 150, 3 port, Pinasco 177, 24/24Si, 140 Main Jet, T5 4th Gear, 226 Vintage Hot Wing pipe, Front Disk Brake & Johnson Rod.
1992 P200e Arcobaleno - Stock.....for now.