Thanks Chris. Yes, the truck would be on a roller for inspection testing. They check the speedo. The vehicle cannot be traveling faster than the speedo reads or the vehicle fails until it is fixed.

I'll get under and check for the sensor or speedo gear. My previous truck had it on the tailshaft of the tcase. I imagine the Wizard will be similar, but I'll look at everything.

If it is electrically operated, I'm going to seriously think about wiring in a TruSpeed unit for another application. They all work on the same principle as you mentioned, they change the reading before it gets to the ECM. I don't see why it wouldn't work here.