There are four things that could be wrong, the speedo itself, the right angle drives at each end of the cable and the cable itself. Personally, my money would be on the right angle drive behind the speedo as the first suspect. If you remove the driver side glove box, you can easily detach the cable from the speedo. Do that, arrange it so you can see the end and then drive the car around the block and see if the cable core turns as you go. That will identify if the problem is the speedo/upper right angle drive or the cable/lower right angle drive.
Yours
Vern