GTO Axle Identification

Overall Housing Dimensions

Size for the standard  mid-year1966 and later GTO differential is as follows:

          Axle flange to axle flange is 60 15/16 inches.

          Backing plate to backing plate is 55 1/2 inches.

          Spring perch centerline to centerline is 35 inches.

          Shock mount centerline to centerline is 49 1/4 inches.

Subtract 1" from above measurements for mid-year 1966 and earlier

No wider unit should be used, but a narrower unit can easily be used with the bonus of using a standard wheel offset.  (GTO maximum wheel width with standard axle is approximately 8"maximum.  This means the rim is 9" wide from outside rim to outside rim.  In order to fit properly with the stock axle, a wheel with 5" backspacing will give 4" to the outside of the rim from the mounting surface.  A 2 inch narrower axle would allow a normal rim to be used with no offset.)


     Axle Ratio Identification:


     Three different differential cases are used. 

        1) 2.56 and 2.78 ratios;

        2) 2.93, 3.08, and 3.23 rations; and

        3) 3.36, 3.55, 3.90 and 4.33 ratios. 

There was also a stronger 4 pinion spider gear installed on 4-speed cars. 

Codes are as follows:

Note "W" on the code is for standard differential, while "Y" on the code is for locking differential or "Posi" rear ends.


GTO: WB or YB  2.56  (41:16)                     THESE CODES APPEAR ON THE LEFT

                     WC or YC  2.78  (39:14)                    HAND AXLE TUBE APPROXIMATELY 2"

                     WD or YD  2.93  (41:14)                    TO THE LEFT OF THE BRAKE TUBE

                     WE or YE  3.08  (40:13)                     HOLDER, OR ABOUT 6" TO THE LEFT

                     WF or YF  3.23  (42:13)                     OF THE DIFFERENTIAL CASE.  THEY

                     WG or YG  3.36  (37:11)                    ARE ABOUT MID HEIGHT.

                     WH or YH  3.55  (39:11)

                     WK or YK  3.90  (39:10)

                     YL  4.33  (39:9)


     Manual Transmission Gears 


          Muncie transmissions:

                    2.52 Low            "FM" on Pontiac or "FO" on Tempest

                    1.88 Second         identify the wide ratio transmission

                    1.46 Third          There are 2 bands on the input

                    1.00 Fourth         shaft.

                    2.59 Reverse


                     2.20 Low            "FN" on Pontiac or "FT" on Tempest

                    1.64 Second         identify a close ratio transmission

                    1.46 Third            There is only one milled band on the

                    1.00 Fourth          standard close ratio and no bands on

                    2.27 Reverse        a "Rock Crusher" 2.20 transmission.