I got my MPGuino installed and working ok but I noticed that it would jitter a lot at certain speeds. I took the speedometer apart to see how it worked and they are really pretty simple. The housing has a magnet inside that spins and makes a reed switch contact. The yellow/green wire is the vss output wire and the other side of the reed switch goes to ground. On my car there was no pullup on the vss side of the reed switch but the cruise control module was pulling it up enough to make it barely work. The automatic door lock thing has been removed from the car and that is probably what normally pulls up the yellow/green VSS wire to 12V. On a scope the signal was pretty noisy.
So my solution involved a couple of 1k resistors. I ran one from switched 12V to the vss wire. And another from the VSS wire to ground. A bit more than a normal debounce but I wanted a clean signal and without the automatic door locking thing I didn't have a clean 12V on the VSS wire. I recommend to do all Metros the same way just to make sure it has a clean signal.
On my site I finally posted a
complete list of wiring diagrams for 89-01 Metros. On mine I went from a black/white wire on the same connector as the vss on the back of the gauge cluster for 12V and black/blue for ground. Optionally on the back of the cluster there are clearly marked + and - places so the resistors could be soldered to the back of the cluster as well.
That pretty much cleaned up the signal on the scope and it should work on all Metros as long as you verify the wiring. I spun the tires up to 65 and it read nice and smooth with no jitters in the display at all.