Because I'm planning to use the RMZ wheel with my new RMZ front end, I will lose the OEM speedometer's functionality. This is because the RMZ is not a road-legal bike, and therefore, the front axle/hub has no speedometer drive gear, as is typically found on road-going bikes. So this meant going to an electronic / magnet type speedo solution. A big advantage to this is that most computerized options these days are far more accurate for measuring speed than the old mechanical speedo, plus these high-tech models include lots of extra features: tachometer, clock, engine temperature, shift indicator, and some other stuff. I don't believe it has a voltage gauge.
A friend of mine, who rides a Honda XR650L, bought a Vapor computer last summer and never got around to installing it. He also bought some accessories to go with it: two aluminium mounting dashboards, brackets, and stuff. He gave me a fantastic price of $75, plus the shipping to my door.