GTO Axle Identification
Overall Housing Dimensions
Size for the standard mid-year 1966 and later GTO differential is as follows:
Axle flange to axle flange is 60
Subtract 1" from above measurements for production 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. (A suggested GTO maximum wheel width with standard axle is approximately 8". 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.)
GETTING WIDE TIRES TO FIT:
Bolt pattern is 5 on 4.75" same as Chevy Malibu - so very common wheel. What's not so common is the required back spacing to get the wheel centered in our wheel wells if you plan on running fatter tires in the rear. Mid 1966 to 1972 Rear axle is about 61" axle flange to axle flange (where brake drums butt up against), but early 66's before about January production were only 60". The current axle needs to be measured to verify width, because it plays an important part in determining what rim backspacing is necessary. The later rear axle requires a wheel with between 5" and 5-1/4" of backspacing, while the earlier axles will only need a wheel with 4-1/2" to 4-3/4" backspacing.
Next thing is to get the maximum width out of the back wheel tubs. The inside is easy since a mallet will smash in the inner fender sheet metal above the frame rail until it is slightly further away from the wheel than the frame rail. Even if there is rubbing, it will be on a totally smooth surface. Only the very bottom metal needs to be moved over, and nothing needs to be done up higher. Outside edge is much more work. For some reason Pontiac decided to use a 2" wide lip on the wheel well, and about half this lip has to be removed. After the lip is trimmed back, the only thing the tire can touch is the smooth rolled edge of the inner wheel well about 2" above the lip. Easiest way to properly trim things back is to start by drilling new screw holes through the stainless molding a half inch more to the outside of the current screw holes. Drilling one hole at a time and moving each screw will keep things in line. After all holes have been made and the screws all fit, take off the molding and draw a line from each of the old screw holes to the next - this will be the cutting line for removing about 3/4" of lip. If you cut past the screw holes, there is a very good chance that you will remove the spot welds keeping the inner fender attached to the quarter panel. We have done about a dozen cars cutting up to the old screw holes and never had a problem. This process usually goes pretty fast using a cut-off wheel, but it is terribly messy and really good eye protection is necessary. If you have any doubts, a body shop could handle the cutting and it should only take an hour of shop time. The stainless molding can then be cut between the screw holes to match the fender lip. A good pair of tin snips can be used for this trimming. A little sanding of the edges will prevent future cuts and snags.
When the above is completed, the width of the wheel well will go from about 10-1/2” to exactly 12”. At this point a 275/60R15 tire can be installed using rims with the proper backspacing. This tire mounted on an 8” rim will be approximately 11” wide, leaving about a half inch on each side of the tire on an 8” rim. A wider rim will also make the tire wider. Figure something like .6” increase in width for each inch of rim width increase.
Our GTO’s came new from the factory with a droopy rear end, and things haven’t improved over the years. A 1” spacer installed under the rear springs will bring the car up to level ride height. A 1-1/2” spacer will provide a very mild “California Rake” to the front. Neither spacer will affect ride quality since it is under the spring and won’t change the spring rate.
Axle Ratio Identification:
Three different differential carriers are used.
1) 2.56 and 2.78 ratios
There was also a stronger 4 pinion spider gear carrier 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. The following codes appear on the left hand axle tube approximately 2” to the left of the brake tube holder, or about 6” to the left of the differential case. They are about mid height.
GTO: WB or YB 2.56
Manual Transmission Gears
FM on Pontiac or FO on Tempest identifies the wide ratio transmission. There are two bands on the input shaft of a wide ratio trans.
FN on Pontiac or FT on Tempest identifies a close ratio transmission. There is only one milled band on the input shaft of a standard close ratio trans, and no bands on a Rock Crusher.