Speedometer questions

One problem can be that the speedo cable's ends are not set into the gear into the trans and into the speedo gauge; they can be tricky to re-engage at both ends. Pull the end of the cable at the trans, not the gear itself, at the trans and carefully re-engage the end into the gear. Also, look at the end and the hole in the gear to make sure they are not worn; they should be square. You can make sure the trans end is engaged by having it disconnected at the upper end and moving the car a bit to see it turns. Then to the same at the speedo end; turn the speedo input hole so it will be lined up with the core out of the cable; if not they will often not engage.

If the cable is engaged for sure, then the speedo's do go bad. I have always replaced them; but a few have carefully disassembled and repaired.

And electronic speedo uses a sensor at the trans or at the upper end of the cable. That turns the rotation into electronic pulses.