Oh! Gotcha. That's not the first time - or the last - that I've misunderstood what a person was meaning. Some GM cars have separate motors running the speedometer and the odometer, so it is quite possible the speedo motor is getting a little voltage happy.
I have a small handfull of those around still, but by now I have no idea whatsover of what they fit, although I do remember that Cadillac owners were a lot more picky than any other GM owners...