Tone Rings
My main experience is with Volvos, old and new.
They use an electronic speedo from 86 on. It has a pickup coil in the differential cover, and a "tone ring" around the spider gear housing in the differential. It spins with the ring gear.
The original design used a sheetmetal cage with square holes cut out of it, and made something like 16 pulses per revolution.
When anti-lock brakes were introduced, they needed better resolution to determine if a wheel was going to lock up, so they increased it by 4x, so the ring has 64 teeth (or 4x whatever the count is of "windows" in the old one). The ring uses cast iron teeth, little 1/8 inch tall ridges on a drum of steel pressed (I think) onto the OD of the ring gear housing.
In either case, it's an electromagnetic pickup, making a sine-wave pulse to the speedo head. For a bike, I'm sure using the unit with higher teeth count, with a speedo designed for fewer pulses, would probably give somewhat closer of a speed indication, just because most bike tires (26" anyway) are a lot larger diameter than car tires.
If you wanted, you could do all the math and figure it out for your tire sizes and that of the "donor" vehicle. That said, the speedo probably has a little potentiometer to adjust it for accuracy.
You're going to have to figure out what the speedo or inst cluster takes for connections. Get the book or wiring diagram for the car you pull the parts from.
Consider using a motorcycle speedo? At least the 91-up Harleys and probably much older Japanese bikes used electronic speedos.
Sounds like a fun, if time-consuming, project!
--=={{Rob}}==--
