The Speedometer is mechanically controlled, It's just a cable that rotates from the transmission straight to your cluster. The Speedo cable is in three pieces:
A long cable from the cluster to the tranny tunnel, a short cable from the tranny tunnel to the actual transmission, and a small cylindrical gear inside the transmission. If any of the three parts is starting to wear, there will be play in the cable, any play will produce an inaccurate reading. Check each part. Spin one end, and make sure the other end spins just as well, with no play. Which ever part of the cable gives play is the part you need to replace.