Your vehicle uses a speedo cable to run the speedometer. Remove the instrument cluster and ensure speedo cable is attached properly. Follow cable down to the output shaft of the tranny and ensure it is attached there as well. Inspect for kinks in the cable. If cable is OK then suspect faulty drive gear in tranny (where cable connects inside of tranny). To test accuracy, follow a helpers car on highway at a set speed (ex. 60 MPH). If your gauge shows 60MPH give / take 3 MPH then your gauge is OK. If its off by 4 or more MPH or is jumping around then there is an issue with the cable.