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M21 with a 3:08?


Well-known member
Jun 20, 2001
Valrico, Fl.
1969 Triple Black Convertible: 2014 Crys Red Conv
What would happen if I put a 3:08 rear end in with a close ratio (M21) tranny in a car with the 350/350? I'd like to be able to cruise down the highway. Currently I have a 4:11. Great from light to light, but stinks on the hwy.

You will have to push it to get it moving or get used to changing clutches. With a M-21 3.36 is as high as you want to go.
Mike, Do you feel it's to much wear on the clutch (for normal driving)?

You will have to slip the clutch to get it moving. This will cause hot spots on the flywheel and pressure plate. Plus you will lugging the engine. You won't like it.
4.11 @ 60mph = 3400 rpm
3.36 @ 60mph = 2600 rpm
That's what I'm looking for. Now I have to find someone who would like a 68 BB 3:08 posi and I can make a deal somewhere. Thanks for the help Mike. No one around here real knew what would happen if I installed the 3:08.
Thanks again.

Your welcome. I've been there and made that mistake. Your best option now is to replace the gears in the 3.08 rerar end. It has a 3 series carrier in it so you can use any gears up to a 3.73.
I've got 3.08 with a 4-speed but mine's a 454. Does the difference between the 350 and 454 make a difference in clutch wear? Will I always be eating up clutches? (I've replaced it once already and the one that was on it had less than 20k miles on it.)

While I agree with the advice here about a loss in performance, I don't think you will see any accelerated clutch wear. I have seen your car and it is a real beauty. You don't race it, you don't autocross it, and you don't have any plans on pumping up the engine. From what I gather, you really enjoy taking it out on the road and letting the wind blow through your hair and soak up that awesome Florida west coast sunshine.

In the long run, you may come out far ahead with the 3.08 for improved fuel economy, lower revs on the highway, and less wear and tear on the motor (spins lower RPM at all times).

When you go with the more aggressive rear end units, you gain acceleration potential, lose fuel mileage, and raise the revs on the highway to where it makes you feel like you need a fifth gear all the time at 70 mph.

So, this is really up to you and your driving style. You are only going to kill the clutch faster with a 3.08 if you are racing the car on a regular basis and trying to get the same take off power as a fellow in a like kind and quality car that is running 3.7 or 4.1 gears.


It is common for the big block cars to have that gear. The big block makes so much torque at low rpm, that it does not use spinning it faster. You can accelerate stronger and faster from say, 1,500 rpm than any equally equiped small block. Therefore, no need to gear it out of site. Plus, it keeps your fuel economy (yah, right--with a 454) as high as possible without loosing overall performance. Again, unless you are abusing the car and trying to hole shot it at every red light, this is NOT going to kill your clutch any faster than anything else out there.

Hey, people kill clutches, gears don't!:eek Or is the guns? :crazy
Chris, Thank you very much. I have been going back and forth with this question of what to do. You are right, I don't plan on doing hole shots with the 3:08. A highway gear is what I am after. But I don't feel like replacing the clutch every other year either (even though I can drop it with removing the tranny, exhaust or driveshaft in about 45 min's). Thank you for the comments about the car. I'm looking foward to a day when we can get all the Florida sharks on a little road cruise. Thanks for keeping us post on your progress. It's exciting see your car come to life!

Chris, thank you, you've made my day (to not hear "oh you should change that" :L)...

Barb :w
I made this change on my 350 77 vette. The gas mileage improved and I have not had any excessive clutch wear. First gear in the transmission is low enough to get it going just fine.

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