# 1964 Rear end upgrade 8.2 to bigger gears & posi



## RockyIce (Jun 2, 2018)

My 1964 was originally a 6 cylinder. Now has a 350 and Muncie M20. I have to assume it’s got a really low gear in rear. Not posi. Not a lot of burst or torque off the line. I only drive the car around town so I’d like to upgrade to 3.33 or 3.55 for better acceleration. I’ve read a lot of the other threads here on opinions. Im wondering what the group thinks on best gear ratio? What kit or brand recommended? I’m confused by the different carrier sizes. If I buy a complete internal parts kit can this conversion be done? Likely high 2s or low 3 to mid 3s like 3.33 or 3.55.


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## BLK69JUDGE (Jun 10, 2010)

best to find out what the ratio is first if it were me...then make a decision..

64's are a 1 year diff with a small upper control arm bushing and a couple other anemic
items.... 
wide ratios like 323 336 or 355 gears 355 was the factory non air wide ratio muncie std gear
what diameter tire you running ??
how much freeway driving ?
all depend also 
I would look for a 65 66 rear with 323 gm gears and put a posi in it


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## Mine'sa66 (Oct 30, 2019)

I would expect low gears from a 6 banger, but if you think it's sluggish off the line, maybe it has tall gears? Agree with blkjudge, find out what you have before dropping the cash.
What's your budget?
You could go the luxury route I went and order up a 12 bolt just the way you want it. I went with 3.31 gears, but I'm running a TH400. Since you say just around town driving, I'd lean more towards 3.55 or even 3.73 with the manual. Keep in mind, you go low like that and highway driving isn't going to be it's forte.
These guys are great and a wealth of differential knowledge..
jdrace.com


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## RockyIce (Jun 2, 2018)

Thanks for the replies. So I'm reading all kinds of conflicting stories here and other places about how to find the ratio by spinning the tire and counting drive shaft rotation. For open rear end, is it 1 tire revolution and count driveshaft turns? Or is it 2? Or some other formula? 
Also, when I lifted both wheels off the ground and spin one, the other doesn't spin at all. Isn't it supposed to spin the opposite direction? Or is it normal for it not to spin at all? Could this be a sign of a problem?


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## RockyIce (Jun 2, 2018)

BLK69JUDGE said:


> best to find out what the ratio is first if it were me...then make a decision..
> 
> 64's are a 1 year diff with a small upper control arm bushing and a couple other anemic
> items....
> ...


Running 27" tires and not doing any freeway driving right now. Probably want to the option to get on the freeway but majority of driving 0-60mph. 
I'm thinking 3.36 maybe 3.55


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## RMTZ67 (Mar 12, 2011)

RockyIce said:


> Running 27" tires and not doing any freeway driving right now. Probably want to the option to get on the freeway but majority of driving 0-60mph.
> I'm thinking 3.36 maybe 3.55


I have 400 manual 3 speed with 3:55 gears. It is a beast off the line and then some. 65 is about max on the freeway. And that's the way I like it. Nobody's gonna pull up and challenge you on the freeway lol.


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## Mine'sa66 (Oct 30, 2019)

RockyIce said:


> Thanks for the replies. So I'm reading all kinds of conflicting stories here and other places about how to find the ratio by spinning the tire and counting drive shaft rotation. For open rear end, is it 1 tire revolution and count driveshaft turns? Or is it 2? Or some other formula?
> Also, when I lifted both wheels off the ground and spin one, the other doesn't spin at all. Isn't it supposed to spin the opposite direction? Or is it normal for it not to spin at all? Could this be a sign of a problem?


Spin the wheel and count the driveshaft turns. It's not the turns, it's the ratio between them. 1 wheel turn with 2 driveshaft turns would be a 2.00 gear ratio. You're only getting close this way, might be tricky to see the difference from 2.56 to 2.73, but you'll know it isn't 3.23. Sometimes it's easier to figure out the ratio by spinning the wheel 2 or more times, might help you differentiate between close gear ratios. Still, it's the relationship between the 2. 2 wheel turns equals 4 driveshaft turns and you still have a 2.0 ratio.
As far as 1 wheel spinning, it's pretty normal for one to spin the opposite way, but if the brakes drag at all, it will just not spin. So no, that doesn't on the surface, indicate a problem.


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## RockyIce (Jun 2, 2018)

RMTZ67 said:


> I have 400 manual 3 speed with 3:55 gears. It is a beast off the line and then some. 65 is about max on the freeway. And that's the way I like it. Nobody's gonna pull up and challenge you on the freeway lol.


Am I thinking about it right that a 4 speed with 3:55s will have a bit more top end speed on the freeway than your 3 speed? or is that not right? I think I want the 3:55 gears but also have flexibility to run 65 or 70 on the freeway for short distances.


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## Old Man Taylor (May 9, 2011)

RockyIce said:


> Am I thinking about it right that a 4 speed with 3:55s will have a bit more top end speed on the freeway than your 3 speed? or is that not right? I think I want the 3:55 gears but also have flexibility to run 65 or 70 on the freeway for short distances.


No. Both transmissions have a 1:1 high gear, so freeway RPM will be identical.


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## Old Man Taylor (May 9, 2011)

The M-20 basically puts a gear between 1st and second of the 3 speed. That's why 1st to 2nd is so close in the 4 speed, and why there is a big drop in RPM from 3rd to 4th.


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## BLK69JUDGE (Jun 10, 2010)

tooth count for the ring and pinion gear ratio is stamped right on the edge of the ring gear on Pontiac GM GEARS

eezee sqeezee


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## Scott06 (May 6, 2020)

In my experience on older cars without OD 3.23 - 3.55 is a nice blend of acceleration and highway cruising. My 65 GTO had 4.33's in it when I got it, was ridiculous. Swapped them for a set of 3.08's haven't driven it much but seems more to my liking. I swapped them myself really wasn't too bad a job once I got the rear out from under the car. I was doing this as part of a suspension rebuild and rear sway bar install so I had to pull the rear anyways. If you do have something lower than a 3.08 you'll need to swap the carrier as well. I bought the install kit off Ebay from Ron's Machine Service. He was a quick ship and helpful with tech support


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