1) The speedo cable is dry and /or binding in the housing.
2) Cruise control? Does the speedo cable run through the cruise control diaphragm?
3) the speedo gear on the trans is dead (or near dead) is there an electric pickup on the speedo gear housing?
4) The speedo head is dead or near dead.
5) If there is a speedo correction device, it could be bad.
Gene