Answer:
More info would help here. There is a cable that runs from the back of the speedo, to the trans. There is a cable running through the outer case of the cable. The core spins, which activates the speedo head. Disconnect the cable at the trans, and spin the core with your fingers, while someone whatches the speedo. It should jump, when you spin it. No jump, bad cable, or speedo. If it jumps, then the plastic gear it connects to inside the trans, is bad. Chevy dealer has them. They are color coded according to rear gear ratios. If the speedo is bad, a speedo shop, or, a reproduction company like Classic ind., or Year 1, will help you.