# 1966 GTO Drivetrain



## Dunn Rite (Aug 19, 2009)

Hello Everyone, I am new the the forum and look forward to learning. I have to pick some brains. I just picked up a 1966 GTO. Its all original numbers matching 389 4BBL, auto. My question is...It has the factory rear or 3.23 open (car has factory air). I want to put an eaton posi unit in it, new axles and switch to 3.55. Is this a good idea? I am pulling out the orginal trans and putting in a turbo 400. I was told the car is 10.75:1 compression ratio. I am looking for help on finding 28 spline ring and pinion, summit only sells 27 spline. I appreciate everyones help.


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## 05GTO (Oct 6, 2004)

Dunn Rite said:


> Hello Everyone, I am new the the forum and look forward to learning. I have to pick some brains. I just picked up a 1966 GTO. Its all original numbers matching 389 4BBL, auto. My question is...It has the factory rear or 3.23 open (car has factory air). I want to put an eaton posi unit in it, new axles and switch to 3.55. Is this a good idea? I am pulling out the orginal trans and putting in a turbo 400. I was told the car is 10.75:1 compression ratio. I am looking for help on finding 28 spline ring and pinion, summit only sells 27 spline. I appreciate everyones help.


You should have 28 spline axles and a 27 spline pinion on your 8.2 10 bolt rear end, I personally think you would make a mistake modding the drive train of a numbers matching GTO. But, thats just my humble opinion!

here is a good place to find ring and pinions; Good luck,

Randy's Ring and Pinion


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## geeteeohguy (Feb 2, 2008)

I agree with 05 100%. I would not modify the drivetrain. With A/C, the 3.23 is about as steep of a gear as you would want, anyway. You'll generate more heat, wear, and fuel consumption with the 3.55 gear. There's a reason the Pontiac engineers did not put a 3.55 gear in AC equipped cars.


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## 123jboy (Nov 15, 2008)

I agree also. My '66 has factory air, and had a factory 3:08 rear. Years ago I broke it and could only find a 3:90, and I thought it would be cool to have a lower gear anyway. I am also running a turbo 400, and believe me, I hated the 3:90. I just installed a 3:23 and what a massive difference. Plenty of torque on the low end, and like a totally different car down the highway. My advice is stay with the 3:23.


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## geeteeohguy (Feb 2, 2008)

Yeah, until you've lived it, it's easy to read all the vintage test articles, and hotrod magazine articles, and lust after a short gear like a 3.90 or 4.11. All the current mags only sell 3.55 and steeper. Then, when you run one, you realize that the concept of a high-torque big block screaming along at 3600 rpm at 55 mph on the highway sucks. Doesn't work anymore. Useless. In the '60's, when these cars were ordered with the steep gears, they were run hard, fuel was less than 30 cents a gallon, and they traded their cars in every 2-3 years, so who cared? With us, we keep our cars for years, restore them, and USE them.....a lot more enjoyment being able to cruise up to the lake or out to the beach, etc. at 70-75 mph at 3000 rpm or less, and STILL have the ability to scare the bejesus out of yourself....not to mention, now you have a FAST car, not just a "quick" one!!!


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