# Rearend quesiton



## mrvandermey (Jun 15, 2011)

Hey guys, I have another question. I am in the process of putting together my chasis. Currently my GTO has a a 10-bolt rearend. Occassionally I see folks descrbing their car with a 12-bolt posi rearend. However, in all the parts catalouges I only see 10-bolt posi rearends. When I see a 12 bolt rearend for sale, it almost always is out of a Chevelle and not a GTO. So my question is, what rearend came with the 1968-69 GTO? Could you get either a 10-bollt or a 12-bolt rearened from the factory in '68-'69? Or are the 12-bolts just for Chevy and/or after market for GTO?


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## GTO JUDGE (May 14, 2005)

10 bolts were the standard. In 1970 12 bolts were offered but where Chevy rears. Pontiac never made a 12 bolt rear.


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## BearGFR (Aug 25, 2008)

GTO JUDGE said:


> 10 bolts were the standard. In 1970 12 bolts were offered but where Chevy rears. Pontiac never made a 12 bolt rear.


:agree True. 12-bolts were available beginning in 1970, and only on GTO's with 455's as far as I know. 

How much power/torque are you planning to be carrying with your car and how will you be driving it? For "general" use with "more or less" stock power, a well built and properly set up 10-bolt should be fine, especially if you update the internals with an aftermarket limited slip diff like an Auburn. 

If you get serious on power, improve the suspension to be able to plant it, and plan to drive "in a spirited fashion" , you might need to consider something stronger like a 12-bolt or a 9-inch. If you need it, and do choose a 12-bolt, bear in mind that most sanctioning bodies require 12-bolts to have c-clip eliminator kits, and the older-style kits with ball bearings weren't recommended for street use. The newer ones with roller bearings are supposed to be ok, as I understand it. (I'm running a Moser 9-inch in the beast)

Bear.


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## mrvandermey (Jun 15, 2011)

BearGFR said:


> :agree True. 12-bolts were available beginning in 1970, and only on GTO's with 455's as far as I know.
> 
> How much power/torque are you planning to be carrying with your car and how will you be driving it? For "general" use with "more or less" stock power, a well built and properly set up 10-bolt should be fine, especially if you update the internals with an aftermarket limited slip diff like an Auburn.
> 
> ...


I am not building a race car. I want a nice boulevard cruiser. I expect no more than mid 400's in HP. Yes, I might partake in a bout of friendly competition from time to time, but mostly I just want to drive it.


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## GTO JUDGE (May 14, 2005)

A 12 bolt would be overkill for what you want. What gears are in your rear?


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## mrvandermey (Jun 15, 2011)

GTO JUDGE said:


> A 12 bolt would be overkill for what you want. What gears are in your rear?


I honestly do not know what gears I currently have, but I would like 3.55 to 3.90 in my final build.


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## Eric Animal (Oct 28, 2007)

If you are gonna drive on the highways, I wouldn't go higher than 3.55:1....unless you add an overdrive transmission......IMHO. eric


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## Knuckledragger (Nov 25, 2011)

BOP 10 bolt differentials are not anything like Chevy 10 bolts. The pinions are slightly smaller than the 12 bolts, 8.2 vs. 8.5 (8.7?), but the Pontiac axles are actually larger diameter than 12 bolt Chevys. Stay stock and don't worry about it.


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## geeteeohguy (Feb 2, 2008)

For a nice boulevard cruiser, I would recommend a 3.36 gear as the steepest gear you would want for a street driven car. 3.23 would be even better. I'd recommend actually driving a GTO with a 3.90 or 3.55 rear end before making the plunge. They are great fun around town, but are terrible at speeds above 60mph. Just my personal experience....unless, of course, you have an overdrive trans!!!


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## GTO JUDGE (May 14, 2005)

mrvandermey said:


> I honestly do not know what gears I currently have, but I would like 3.55 to 3.90 in my final build.


You can verify the gears the old fashioned way if you want to know what you have. You may already know this but: Elevate the wheels off the ground. Place something on the ground beside the face of the wheel, this will be your reference point. Line up the valve stem so its inline and straight up from that reference point. Have someone watch the drive shaft/yoke. Have them count how many times the drive shaft spins to one complete revolution of the wheel. If the drive shaft turns say 3.5 times to one full turn of the wheel you'll be close to 3.55's nearly 4 turns you'll be close to 3.90's. 

I'm sure you are aware of this method but if you aren't this will give you an idea of what ratio you have. Many stock GTO's came with 3.23 gears which like gee said is great cruising gears and easier on gas. 3.55's is perhaps the most popular with good highway cruising and plenty of oopmf for getting on it when you feel like it.


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## mrvandermey (Jun 15, 2011)

GTO JUDGE said:


> You can verify the gears the old fashioned way if you want to know what you have. You may already know this but: Elevate the wheels off the ground. Place something on the ground beside the face of the wheel, this will be your reference point. Line up the valve stem so its inline and straight up from that reference point. Have someone watch the drive shaft/yoke. Have them count how many times the drive shaft spins to one complete revolution of the wheel. If the drive shaft turns say 3.5 times to one full turn of the wheel you'll be close to 3.55's nearly 4 turns you'll be close to 3.90's.
> 
> I'm sure you are aware of this method but if you aren't this will give you an idea of what ratio you have. Many stock GTO's came with 3.23 gears which like gee said is great cruising gears and easier on gas. 3.55's is perhaps the most popular with good highway cruising and plenty of oopmf for getting on it when you feel like it.




Now I may be comparing apples and oranges here, but I had 3.55 gears on my 2005 Mustange GT convertible, and I liked them. 

No, I did not already know about this testing method, I will give it a try.


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## BearGFR (Aug 25, 2008)

mrvandermey said:


> Now I may be comparing apples and oranges here, but I had 3.55 gears on my 2005 Mustange GT convertible, and I liked them.


Yeah but... your Mustang probably had an overdriven high gear, your GTO doesn't. I had 3.73's in a 90 5.0 5 speed and they were great, highway driving was still very nice due to the overdrive in 5th.

With 3.50's in my GTO and the 10" converter, 70 mph is about 3200-3300 rpm or so, whereas 70 mph in my Mustang was around 1800-2000, even with the 3.73's.

Bear


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## GTO JUDGE (May 14, 2005)

For comparisons: With 3.55's in mine I am taching at 28-2900 at about 60-65mph. 66-75mph I am at 3000. When I had 4.10's I was tacking 3000 at 55mph. ouch.......


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## geeteeohguy (Feb 2, 2008)

My '65 GTO is right at 3000 rpm at 70mph with stock height (26.5" diameter) rear tires. That's with a 3.36 rear gear. Another thing that should be considered is this: Mustang 5.0's and Camaros have high winding, small block engines. They _need_ a stiffer gear to get going due to lack of torque. A Pontiac doesn't need a short gear to get up and go....it has plenty of torque. Stiff gears not needed, and for me, not desired at all. 45 years ago, when 100 octane was 29 cents a gallon and no cars had overdrives, steep gears were not an issue. Dragstrip E.T's sold cars. Today, with $4 gas and everybody spoiled with overdrives and fuel economy, it's pretty nice to have a high performance Pontiac that you can actually take on road trips!!! I'll take low engine wear, great fuel economy, and low road noise over bragging rights for a stiff gear any day. But that's only because I drive my cars. (I'll get off the soap box now.......)


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## Chips (Nov 26, 2011)

Does anyone know or know the best source to find-out the casting numbers for rears used in 68/69 GTOs?


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