[Terry M. Hunt]

Converted my 66 GTO from auto to 4 speed and still have the 3.08 rear axle in place. Anybody else running this combination and what are the pros and cons?

[padgett]

Pro: MPG (but really need a 2.56 with a M20 to get 2000 rpm at 70).
Con: Frequent clutch replacement

Back in the day, there was a Saginaw 4 speed with a 3.31 first gear but kinda marginal on torque.

[bdk1976]

Have you driven your car? Why don’t you tell us?

[PontiacJim1959]

Originally Posted by Terry M. Hunt

Converted my 66 GTO from auto to 4 speed and still have the 3.08 rear axle in place. Anybody else running this combination and what are the pros and cons?

Bad combo. Factory tires were 26" and often newer tire can be taller, effectively dropping that 3.08 down into upper 2's.

Taking off will require a lot more than normal clutch slipping to get rolling, especially on a stop at a grade or steep hill. So you can expect premature clutch wear and replacement - and maybe some embarassment when you stall the car on a steep hill.

Most wide ratio manual trans cars from the factory will run a 3.23 and up with 26" tires. It is doable, and I have not knowing much about cars in my youth, but I also was replacing clutch discs about every 3 months with the way I drive - and I figured I was just buying NAPA brand discs that were junk. Now I realize it may have been the rear swap from the close ratio M21/3.90's to 3.08's.

[The Champ]

I switched from 3.42 gears to 3.08's in the 12 bolt in my '64 GTO with the original M20. Effective 1st gear is nearly 7.88 - very similar to how it left the factory with the original 3.23 gears which had an effective 1st gear of 8.26. I drove the car with 27" tires for 20 years with 3.23, 3.36, 3.42 and 3.08 gears..

Original factory tires for a '66 GTO were actually a little over 27".

https://www.cokertire.com/tires/750-...8-redline.html

It works fine and RPM's at 70 MPH is about 2700.

[Terry M. Hunt]

I did the swap years ago and barely drove the car before sticking it in storage so I don’t recall how it drove. I noticed in the Restoration Guide on pages 310 and 311 charts showing all the rear gear/transmission combinations offered in 1966. For cars with standard brakes, a 3.08 ratio was only available with auto trans and A/C. For cars with metallic brakes, a 3.08 ratio was available with a 3 speed, a 4 speed, or an automatic with or without A/C. The chart does not seem to specify whether the 4 speed was a close ratio or a wide ratio. If this combo was so hard on clutches, it seems surprising that it would be offered by the factory. Maybe I’m missing something.

[Goatracer1]

A friend of mine ran a 3.08/ 4 speed in a 1972 Ventura 2 which should be close in weight to your car with no problem. Give it a try.

[bdk1976]

Originally Posted by Terry M. Hunt

I did the swap years ago and barely drove the car before sticking it in storage so I don’t recall how it drove. I noticed in the Restoration Guide on pages 310 and 311 charts showing all the rear gear/transmission combinations offered in 1966. For cars with standard brakes, a 3.08 ratio was only available with auto trans and A/C. For cars with metallic brakes, a 3.08 ratio was available with a 3 speed, a 4 speed, or an automatic with or without A/C. The chart does not seem to specify whether the 4 speed was a close ratio or a wide ratio. If this combo was so hard on clutches, it seems surprising that it would be offered by the factory. Maybe I’m missing something.

The wide ratio Muncie (M20) went with 3.08. I have a '65 GTO that is an original M20/3.08STT. I have NOT driven my car as it is third in line for resto. I expect to have enough torque with my engine to make it a moot point, and being able to wind it out to like 50mph(?) or something crazy in first gear is kind of cool...lol

[GoGoat]

My 66 stock tripower/ 4 speed had high 2's when I got it. Hardly got out of 2 nd gear in town. Had to shift to 1st at corners. Went to 355's. Much better. All depends on driver needs, type of use, terrane. and future uses. Good luck.

[The Champ]

Originally Posted by Terry M. Hunt

If this combo was so hard on clutches, it seems surprising that it would be offered by the factory. Maybe I’m missing something.

You're not missing anything.

With the wide ratio M20, a 3.08 rear gear is perfectly fine for regular driving. It may not be the preferred rear end for doing burnouts or the drag strip, but I don't think that's your objective.

[geeteeohguy]

Originally Posted by The Champ

You're not missing anything.

With the wide ratio M20, a 3.08 rear gear is perfectly fine for regular driving. It may not be the preferred rear end for doing burnouts or the drag strip, but I don't think that's your objective.

Agree. This was a factory option and wasn't an issue back then and isn't now. These cars have plenty of low end torque to make it easy to start out with an M20 and the 3.08. Not even an issue. BTDT.

[JLuger]

I'm running a M20 2.73's with a trutrac stock 455, 235/60/15's in my 66 Goat
From everything I read thought for sure I'd have chatter, Nope.. you can roll the clutch right out I drive the car all the time and everywhere its not a burn out machine but is a great all around driver.

[geeteeohguy]

Originally Posted by JLuger

I'm running a M20 2.73's with a trutrac stock 455, 235/60/15's in my 66 Goat
From everything I read thought for sure I'd have chatter, Nope.. you can roll the clutch right out I drive the car all the time and everywhere its not a burn out machine but is a great all around driver.

Not a burn out machine, but I'll be you enjoy cruising 80 mph, sipping fuel, and running cool. Instead of buzzing along with a hot engine at 60 mph in the slow lane getting passed by garbage trucks, but being cool because you have a 4.11 rear end!

[JLuger]

You know I never thought about that contributing to the low temps the car runs 180 on a 90° day might see 200° If I drive it hard that’s with stock factory cooling setup and factory exhaust with resonators if you can believe it.

[gtorich]

Originally Posted by geeteeohguy

Not a burn out machine, but I'll be you enjoy cruising 80 mph, sipping fuel, and running cool. Instead of buzzing along with a hot engine at 60 mph in the slow lane getting passed by garbage trucks, but being cool because you have a 4.11 rear end!

That was funny............i ran a 455 with a M-20 ...3.08 for about 10 yrs, no issues, switched to a 3.42 rear, ran good, but not really all that much difference for the money spent.

Rich

[JLuger]

I was going to find a set of 308 gears for it and I had another 10 bolt with 3.55’s I had ran in another Gto this car came with a 8.5 10 bolt and the 2.73 gears where like new so I gave them a try.

[bhill86]

I converted my 77 T/A from auto to a ST10 with a 2.41 rear. That was less than ideal. Realistically I should’ve looked into a Richmond 4+1 but instead I swapped to a 3.08. Way more practical than the 2.41/4 speed setup. Still highway friendly but not a dog either. I also don’t feel like it’s overly hard on the clutch to get rolling. The 2.41 though did feel that way.

[tom s]

I have run the ST-10 3.42 first gear trans in a few diff cars.Now in a 62 GP with 421HO and 2.68s in the back.Great combo for driving,would not try to beat on it with big sticky tires.Tom

[Andre]

The beauty of the Pontiac V8 is that it has more than enough low end torque so that gears like 2.56:1 will still burn the tires. Case in point being my old 1964 conv clone with stock never taken apart 389 and 2 speed auto with 2.56 rear and 14" tires. It would smoke these tires with no effort, yet ran cool as a cucumber (just at 180) at 2000 rpm at 60mph. I think the new owner loves it more than I did.

[scott06]

Originally Posted by Terry M. Hunt

Converted my 66 GTO from auto to 4 speed and still have the 3.08 rear axle in place. Anybody else running this combination and what are the pros and cons?

I just swapped 4.33's for 3.08s in my 65 gto with a 4 spd.

The answer depends on what you want to do with the car.

I just want a cruiser so it works great and keeps highway rpm down. if you want something snappier try a 3.55 or 3.73. I've had other cars with a 1:1 4th gear and 3.55 they are pretty good at highway speed