# Rear Diff Gearing (67 GTO)



## Jim (Oct 30, 2008)

Well, I need some help, again.........
I bought my car recently, site unseen (I'll never do that again) and everything was supposed to be fine. Firstly, I had to replace the clutch which we all talked about (clutch, pressure plate, flywheel, etc.) The next thing I have been dealing with is it seems to have a problem out of the hole like the gearing is to high. Several of you offered some great advice of determining what rear end is in the car. I got under there tonight and did the tire spin/drive shaft rotation to determine what gearing iis in the car. As far as I can tell it's a 2.56 and the car is supposed to have 3.55 based on the build sheet. It also has a 10 bolt rearend. I don't even think it's the original rearend. 

Did the car come with a 10 bolt or a 12 bolt rearend?
If I have the gearing changed what do you think is a fair price?
While I am at it do you think I ought to have posi put in?

Thanks for the input.

Jim


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## Rukee (Feb 8, 2007)

All GTOs came with a posi, so that rear end or at least the carrier must have been changed along the way somewhere. I would do an Auburn limited slip diff and the 3.55 gears. Call the guys at Randy`s Ring and pinion and they will fix you right up with the parts and install kit you need.


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## geeteeohguy (Feb 2, 2008)

I respectfully must disagree about all GTO's coming with a posi (Safe T Track) rear end. My '67 GTO has an open 3.36 original diff, and many, many GTO's came with open diffs. Safe-T-Track was about a $43 option. It was not standard. That being said, until '70, all GTO's had the ten bolt rear end. In '67, the standard ratio with a 4 speed was 3.55. The 2.56 is the "correct" unit, as it looks and is constructed exactly like the 3.55 unit, except the ratio is all wrong for a stick car. 2.56 was common in Tempests for economy. Great hiway gear, if you have an automatic trans. In a stick car, you want at LEAST a 3.08, but a 3.23 or 3.36 or 3.55 would be waaay better. I fully agree with the Auburn posi that Rukee recommends, and his course of action. He's been there/done that on a lot of this stuff. Good luck. 
PS: read the top thread on the Forum titled Gear Ratios. 
Jeff


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## 05GTO (Oct 6, 2004)

Here is a listing of GTO rear ends;

Pontiac Rear Ends


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## Rukee (Feb 8, 2007)

I love it when I stand corrected!! 

I was under the impression that the posi was part of the GTO package.


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## geeteeohguy (Feb 2, 2008)

If it makes you feel better, I thought the exact same thing for years until I got Ed-u-ma-cated. When I found out, It was like finding out the Easter Bunny was a sham! But, I did eventually recover. The funny thing is, the longer I deal with these cars, the less I seem to know, and themore I seem to learn. Also, I appreciate them more and more each year: much more than when I was driving them every day 25 years ago. Time to fire up the '65 and put some fuel in 'er: Premium is down to about $2.05 here, the lowest it's been in about 3 years!!!


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## Jim (Oct 30, 2008)

Jeff,

What diff gearing do you have??

Jim


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## geeteeohguy (Feb 2, 2008)

Jim---I have 3.36 gearing in both my GTO's. My '65 is a 4 speed car and my '67 is an automatic. My '65 had a 3.90 gear (It came new with a 3.55 posi) and I changed the ring and pinion to 3.36. It cruises nicely at 70mph and has excellent acceleration. My '67 has an automatic and has its original 3.36 open rear end. My favorite rear ratio is 3.23 for a stick car: good cruising at high speed with minimal noise and engine wear, good gas mileage. I had a 4 speed '65 with a 3.23 and it was my favorite. The 3.36, though longer legged than a 3.55, still feels like I need a higher gear at high speeds. For your '67 with stickshift, I recommend a 3.23, 3.36, or 3.55 in that order. You can use the 2.56 carrier you have if you change to 3.23 gears. If you want to go to 3.36 or 3.55, you will need a new differential carrier. The 2.56--3.23 ratios used a "3" series carrier, the 3.36-4.33 cars used a "4" series carrier.


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