2 things to check

1.check the speedo cable and the vss for any mechnical problems cable not in all the way vss missing. it mechanical so no electrical checks. if everything is good then go on to number 2



2. the speed pulser you check that by taking out the gauge cluster and sticking a pencil in the back of the speedometer cable connecter socket and then turn the pencil while checking resistance on the two terminals below it (they look like screws) there will be resistance at 4 points while turning the pencil 360 degrees. if there is no resistance you will need a cluster.