I've seen people use bike speedo's with good success. ONE caveat is that you may be getting electrical interference from the ignition system. I've seen this be a problem with motorized bicycles. Make sure the magnet gap is tight, and that the wires are not too close to the ignition system, and that you are running a resistor plug.
A way to check would be to run the bike to speed, kill the motor and pull in the clutch. If the speedo works correctly that's your problem.
