A speedometer cable has a thick outer casing, and an inner part that spins.
Here's some info from the Mk2 VW Service Manual:
http://eva2.homeip.net/eva2/BD01/ch4.1.1.html
The drive gear has a different number of teeth depending on the final drive / differential ratio of the transmission. This results in the same number of rotations of the cable for all transmissions.
-J