Gerry,
I have had a sticking speedometer cable break the speedometer while the odometer kept working, for a while anyway. The symptoms would be noise from the cable as it sticks, winds up, then breaks free, with the speedometer pegging the dial. Broke the needle off.
Anyway, there is a fairly delicate little mechanism around the needle, and I seem to recall that I also had one that got stuck. It's not that hard to take one apart enough to determine if it is stuck.
I took one to a local speedometer shop which fixed it, and sent one off for repair. In neither case did I feel confident that I could fix it myself, because I had no info. This was in the pre-internet days.
I realize my comments are not much help, but maybe they will help get the conversation started.
Kurt
