just a bit of info on VDO speedos...

The cable will break. The only way to know if the cable is turning is to watch it. I usually pull out the instrument panel and take a friend along for a trip around the block. They watch to see if the cable turns. If not then replace it.

Cable is turning? Then the magnet inside the speedo needs to be checked for movement. You have to remove the speedo, which is just a few screws or bolts, from the instrument panel, and then study it for a bit to understand how to remove the back cover. Once the cover is off see if the magnets are rotating freely. If not then they are frozen and nothing will unfreeze them to the point that your speedo will ever be accurate. Therefore it is time to get a new one or at least a working one.

If the magnets are turning then spin them around and see if the speedo registers any speed. If not then chunk it and get another one since they are too hard to tear into and too easy to find on Ebay.

If the speedo is working and the odo is not, which is usually the case, then do a web search for 'replacing odo gear'. This is easy and cheap.