The easiest thing to do is open Google Maps on your phone. Enter a destination and start driving. It shows your actual speed as you drive. I recently used this for my Galaxie 500 and discovered that the speed and miles are registering exactly 12.5 % low at all speeds.

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)