if it was off the same amount with the original trans, it's unlikely to be the wrong gear .... more likely that the speedo mechanism is gummed up after 55 years, and if the speedo has ever been messed with it could just be the needle is mis-positioned - notice whether it starts to move as soon as you let out the clutch or 'jumps up' once you're moving faster [or drops to 0 before you're stopped]
Bill
