Speeedometer

Well starting at the transmission you could have a bad speedometer drive gear, or a bad cable on up to a bad speedometer head.
Most likely is a bad cable. They are not hard to change, the bottom end unscrews, the top plugs into the back of the speedo head in the dash cluster. I have changed some cables without removing the cluster, it all depends on if you can reach the clip that holds it in place on the stem on the rear of the gauge.