# Trans/rear end help



## 3Boyz14 (2 mo ago)

First of all, even though I’m 61 years young, I’m new to the GTO scene, so please be gentle. I recently purchased a 66 GTO coupe that needs a total restoration. The car came from the factory with an automatic transmission and 3.08 gears with the posi narrow rear end. I have a 389 tri power engine with a mild cam and M 21 close ratio to go into the car. From what I have read the lowest gear ratio I can install in the 8.2 rear is a 3.36. My question is will the 3.36 give me any low end torque? I’m old but still like to burn a little rubber, and would like to be able to bark a tire in 2nd, every now and then. I plan on mostly driving around town and taking a few trips out of state. Thanks for any feedback.


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## NYTrainer (Sep 19, 2012)

When a close ratio Muncie was ordered it came with either 3.90 or 4.33 gears (at least that’s the case on ’68’s). 2.20 first gear and 3.36 gears will give you a 7.4 which can be tough on a clutch especially in hilly areas. An M20 (2.52 first gear) will give you 8.5 overall first gear. Maybe someone with the combination you’re suggesting will chime in with their experience.


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## 3Boyz14 (2 mo ago)

Thanks. Looks like I’m going to have to find a m 20


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## lust4speed (Jul 5, 2019)

The lowest rearend gear offered in a BOP 10-bolt rear was dealer installed 4.33's. That required a carrier that accepted gears from 3.36 up to the 4.33. Your carrier will only go one more ratio to 3.23 (your carrier will hold 2.93, 3.08, and 3.23 gears). All the new aftermarket limited slip units are for the 3.36 and lower gears with the most common gear available being the 3.55. A close ratio really needs at least a 3.55 gear to have a decent first gear with the 2.20 low in the trans. If you keep your 3.08 (which is a good highway gear) then the wide ratio Muncie with the 2.52 low will help, but still be a pretty high first gear. You will have 7.76 with the wide ratio and 3.08 rear, and 7.81 multiplication with the close ratio and 3.08 gear. A really comfortable ratio is 9:1 but not necessary. My 3.36 rear with the 2.52 first was 8.47:1 and that did okay for 55 years and many miles.


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## PontiacJim (Dec 29, 2012)

3Boyz14 said:


> First of all, even though I’m 61 years young, I’m new to the GTO scene, so please be gentle. I recently purchased a 66 GTO coupe that needs a total restoration. The car came from the factory with an automatic transmission and 3.08 gears with the posi narrow rear end. I have a 389 tri power engine with a mild cam and M 21 close ratio to go into the car. From what I have read the lowest gear ratio I can install in the 8.2 rear is a 3.36. My question is will the 3.36 give me any low end torque? I’m old but still like to burn a little rubber, and would like to be able to bark a tire in 2nd, every now and then. I plan on mostly driving around town and taking a few trips out of state. Thanks for any feedback.


The M21 is a desirable transmission being a less common close ratio unit. Basically what it boils down to is the first gear ratio coupled with the 3.08's will be very difficult to get rolling ad require a little clutch slipping to do so, even moreso on ay kind of hill. Clutch life will be greatly shortened.

Have you considered the Tremec TKX 5-speed? You could keep the 3.08's. The TKX offers different 1st gear and 5th gear ratios. Go with the 3.27 first gear and .72 OD. No issues taking off and you can cruise on the highway (28" tall tire) at 2,000 RPM's @ 72 MPH.

Click on the link below for the Tremec Calculator:



https://www.tremec.com/calculadora.php


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## geeteeohguy (Feb 2, 2008)

If I had a '66 GTO that was a 2 speed car and I was converting it to manual, I'd install an M-20 or even a Borg Warner or Richmond Gear ST-10 trans with the 2.66 first gear and leave the 3.08 in place. If I had a 3.08 I would NOT swap in anything but a 3.23 on a stick shift car with no overdrive, and the way I drive mine, I would leave the 3.08 in. 3.08 was a factory ratio with these cars with a M-20 or HD s speed when new. It is not an issue to pull with even a 215 6 cylinder. It's the longest legged rear you really want to run with a stick, but will be good on gas and for low RPM cruising. Like the OP, I am also 61 years, but old, with the exception of driving these cars since the '70's non-stop as primary transportation. I have experienced all types of ratios in these cars from 2.56 to 4.56, and on a street driven car that actually may see 70 mph speeds on the highway for more than an exit ramp, you want gears in the high 2's or low 3's. Grinding along at 3000 rpm at 55 mph in the slow lane running hot and chugging gas while being passed by Nissan Leaf's sucks. The Richmond gear 4 speeds brand new are about 2k, and IMO, are a better unit than an old Muncie that may be beaten or worn badly. And they give you the option of several gear ratios so you can cheat and run a longer legged rear end without sacrificing in-town performance. 
Keep in mind with these 55+ year old cars, banging gears and burning rubber can break parts with modern sticky tires. I have blown engines, transmissions, and rear ends on these cars, reducing them to scrap metal. That was when a quick trip to the wrecking yard could get me replacements of all three for under $400 combined. Last thing I blew up was the rear end on my '65 in 1990. I haven't messed it up or broken anything since. Fingers crossed.


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## lust4speed (Jul 5, 2019)

I need to correct my math on my #4 post above. My proof reading and math were out to lunch. The ratio for the 3.08 and 2.20 close ratio is a meager 6.78 and not the 7.81 I misstated above.


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## geeteeohguy (Feb 2, 2008)

Agree that a 3.08 with a 2.20 first gear is far from optimal. But a 3.08 with a 2.52 first gear is not an issue. Lots of Pontiacs left the factory this way.


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## 3Boyz14 (2 mo ago)

Thanks for all the info, I really appreciate everyone’s input. I’m thinking about going a different route. After a closer examination of the auto transmission I pulled out of my car, found it’s a TH 200 4R. It should have a 2.74 ratio 1st gear which will work good, and I’ll just put a shift kit in it ( for my 2nd gear tire bark)


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## chrisn (2 mo ago)

i would stay away from the tkx with the 327 low gear . i have a world class t5 in my cobra with the same low gear with 3.55s out back and low gear is good for 15 feet before shifting to second im looking to change to a tko600 because of its taller low gear 2.87


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## geeteeohguy (Feb 2, 2008)

chrisn said:


> i would stay away from the tkx with the 327 low gear . i have a world class t5 in my cobra with the same low gear with 3.55s out back and low gear is good for 15 feet before shifting to second im looking to change to a tko600 because of its taller low gear 2.87


I have a 3.42 low gear ST-10 in my '61 Corvette with a 3.36 out back and the same thing. Good for parades or pulling stumps. Second is 2.20, which is low gear on a M21 Muncie!
I shift from first to third all the time and it sounds totally normal. Wish GM made a 2.75 gear for the old solid axle Chevies.


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## Baaad65 (Aug 29, 2019)

chrisn said:


> i would stay away from the tkx with the 327 low gear . i have a world class t5 in my cobra with the same low gear with 3.55s out back and low gear is good for 15 feet before shifting to second im looking to change to a tko600 because of its taller low gear 2.87


The TKX comes with a 2.87 first gear also.


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