Your issue has a easy fix. The piece that the cable attachs to on the trans is removable. It has a plastic gear on the end of it that determines the rotation speed of the cable, and by changing that out with the correct color plastic gear will correct your speedo.
The problem is, you need to determine which one you need. Once you pull the speedo gear out of the trans and see what color it is (also the tooth count) and find out what rear end gear you have, then you can find out what color (tooth count) plastic gear you need to put back in the trans for your speedo to be closest to perfect....
There is a list floating around somewhere with the color/gear count combos that show which one should go in which trans/ rear end combos, but I don't know where to find it at the moment.