speedo doesn't work
 

I picked up a clean 93 hatch on Tues. and i knew that a few things didn't work, nothing big but thats pretty much why it was so cheap.

The main thing i need to fix is the speedometer, it lays all the way flat to the right and doesn't move till highway speeds then goes backwards to 90. what could the problem be? something like this happen to anyone else? Also if it is the actual cable, could i swap out the one from my auto 90, not sure if they're compatible?