Gear Ratios

I have a co-worker who has a 1965 Pontiac GTO convertible with the wrong rear end in it. Could anyone help us find out what the original gear ratio was? It is a 4 speed, standard. Any information would be greatly appreciated. Thanks!

HI
65s with manual trans could have been 3.08-3.23-3.36-3.55 or even special ordered with 3.90 0r 4.33. If the axle housing hasen't been changed look for the identifying code stamped on the housing on the right sdie of the carrier. D-3.08 K-3.23 E-3.36 F-3.55. If it had heavy duty rear brakes G-3.08 L-3.23 M-3.36 N-3.55 P-3.90 S-4.33.

What is the rear end in it now?

One way to tell for sure - send $35 to PHS.