It sounds like it is NOT the speedo unit itself. Either different rear end, different tires, different trans. Speedo itself is probably OK. Should be able to correct with right tires or change the speedo gear in the tranny.
68' Firebird 400 convertible, numbers matching, solar red w/ deluxe parchment interior. 66' Pontiac Ventura Hardtop 66' Pontiac Catalina Convertible
