Quote Originally Posted by th62 View Post
........ I've bought a magnetic sensor speedo so I don't have a great heavy cable running down beside the fork.
I have just checked the schematic for my tacho and a speedo will be much easier to wire up. Most of the components in the tacho are for conditioning the noisy signals from the ignition system and you will not need that circuitry. A magnetic or optical wheel sensor will feed directly into the Arduino which feeds directly into the stepper.

Please post a link to the speedo that you bought so that I can check out that the sensor is suitable. Also how do you plan on triggering the sensor? You may be able to do that off the bolts that hold the disc, unless they are non-magnetic stainless. You will get a smoother display with multiple pulses per wheel revolution. It is only a setting in the software to accommodate any number of pulses and the maximum speed that you want to display.