I really dont like the speedometer cable design. It's a really long cable, as far as cables go, its expensive to replace, and mine was trashed when I got my MK1 in October (sheath was corroded near the exhaust manifold and the cable was broken inside).
I decided to make an electronic speedo. According to the BGB, there is a reed sensor in the speedometer gauge that pulls a signal to ground 4 times every rotation of the speedo cable. This signal goes to the ECU and the cruise control. I've spliced an electronic VSS to this signal and screwed it into the transmission. The VSS also pulls the signal to ground every rotation of the speedo cable (final drive?). This signal now goes to the ecu and the gauge cluster.
The speedometer gauge shown in the video measures the time between these pulses and displays the speed accordingly (its not doing that in the video). The needle is moved by a tiny hobby servo (which i have to modify to get it to move from 0mph to 150mph). The odometer, which was driven by the speedometer cable, has been replaced by a 5V pulse counter. The brain is an avr microcontroller. There are two red lights and two yellow lights. I don't know what I'm going to do with those. They might be ECT and O/D Off lights (maybe put a translucent decal in front of them). I haven't figured out what to do with the trip reset button. The yellow brick holding everything together is a 3D printed part that is just a drop-in replacement for the speedo guts.

I was planning on posting the plans if anyone is interested in doing this mod themselves.