I have a bicycle speedometer on my minibike
, and I had some trouble with it:
First, the speedometer got crazy when I started the engine, it was because the ignition was disturbing the meter I guess, because when I fitted a new spark plug contact, that problem was over...
And then the meter went just fine, just up to 50mph

Then, the meter couldn't count the wheel rpm, because I was going too fast and my wheels are too small...