Wobbling or malfunctioning speedometer

The speedometer cable is split into two parts but that doesn't mean you should replace only one part, unless you are positive that only either the upper or the lower part is failing. But even then, the other part may follow its counterpart in a short notice...

Replacing the cable is simple in theory but still might be quite demanding, especially the lower end at the transmission (unless you have an inspection pit or a car lifter, that is). There is a rubber cotter pin perpendicular to the cable which you have to remove first, then the cable can be pulled from the transmission easily. In order to get at the upper end you have to remove the instrument panel.

When you refit the new cable, make sure to avoid bending it sharply. The standard piece of advice is to route it the same way the old cable was routed but take this with a grain of salt: what if the cable was already replaced once and whoever did it didn't pay attention to the routing? So, if you find that a different routing forces less bending, take that one. By disconnecting at the center you can change the routing later, too, especially if you find the speedometer needle wobbling. On right-hand drive cars it is trickier to route the cable, you might need to experiment to find the best setup.

When connecting the two parts, be sure you push them together completely and secure them fully with the nut of the upper cable. If it's loose, the needle will inevitably wobble.

To replace the upper part the best way is to pull the new cable into its place by attaching it to the end of the old one. And don't forget to apply some sort of grease (or even better, a special lubricating spray for rotating cables like that from CRC) before you connect the two parts. No matter what you do the meter needle will start to wobble in due time but you can try to prolong this period of time...

And, by the way, check that the end of the cable rotates when you rotate the other end by hand. I reckon it might be quite rare but I already happened to buy a brand new cable only to find out that it was not working. Certainly, it was replaced by the parts store promptly but that didn't spare me the trouble of replacing it twice...