After 214.000 km the speedometer in my 1988 CX GTi finally began make a
clicking noise. Since the car was for sale this had to be examined promptly
and hopefully also fixed.

This is a picture revealing the reason:

As you can see the green plastic helical gear is worn. With the screw
thread in place it climbs up and down on the screw. It still rotates but the
climbing makes a loud clicking noise every time it falls back into place.

The fix must be