Re: Mechanical speedometer
the biggest problem with those cheap speedos is vibration. they're not designed to hammer down bumpy roads at 30+.
get some rubber washers, or make something to isolate it from shaking around, and it'll work.
the other problem is accuracy. i've seen videos where people claim 40+ speeds, and there's a cow in the background doing 50 (not really, but you can see where i'm going with this.)
calibrate it with a stopwatch, or pace a buddy's car, or else it's just a toy strapped to your handlebars.