If you hook it up the way it is now, you can calculate the percentage difference that it is off by using a GPS on your phone. When that is known, some sellers have a chart to follow in order to get the exact unit you need. The adapter screws into the transmission and the speedo cable screws into the adapter.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)