The cable is likely to long like Wally found, try backing off the connector or adding a spacer between the cable housing and the gear housing. Also make sure the housing is square to the speedometer and gear housing. As a test, pull the cable off the transmission and hook an electric drill to it. Have someone watch the speedometer when you speed up the drill, if the speedometer sets steady, you have a cable length, cable housing, or lack of lubrication problem. If the speedometer is erratic, you need to clean and lubricate it.
A long cable pushes on the speedometer head and messes with the magnetic drive. If in fact it shows 100 mph, then odometer should show 10 miles every 6 minutes.
