How do generic mechanical speedos work? It just senses the speed of the wheel so road speed will depend on the wheel size - is there an industry-standard speedo cable factor that the gearbox on the wheel adapts to?
As for wiring, you'll have to get creative with the turn signal connection (if you don't know what a diode is you probably won't figure it out on your own ) but the rest should be a piece of cake with a diagram
The speedometer drive gearbox at the wheel translates one wheel rotation to X amount of rotations of the speedometer cable.
There are a few different ratios in use but most Hondas use something like 2240=60MPH there is an equal metric ratio that is something like 1440=60KPH but it works out to be the same.
92 CB250 Nighthawk - partsbike
93 CB250 Nighthawk - partsbike
92 CB250 Nighthawk - project
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81 CM400C - Project, on the road
73 CB750K3 - Long term project