Thinking about changing gear ratio and wonder how performace would be affected on a 66.

Engine is a 396 at least .030 or more over, with the original 98cc 702 closed oval heads, with a stock holley intake and stock holley 3419 carb, , the stock 360 hp cam, and stock exhaust manifolds. Not sure about the pistons or the compression ratio, original compression was 10.25:1. Cylinders test out at between 175 & 180 pounds of compression.

Car has a 66 12 bolt with 3:55 gears and possitraction, the transmission is a 66 M20.

I have a set of gm 3:07 and a set of gm 3:31 gears. Wondering how you think performance would be with a change to either of these gear ratios from the 3:55? Thanks.

My car originally had 3:08 or 3:07 and it drove great. They are really a perfect match for a nice street performance big block. The torque makes up for the gear on the lower rpm side very well IMO.

I had a 69 Automatic with a 3:08 and I'll say this.... It was a highway machine.

It just kept pulling and pulling when I hit the gas.

I've got a 3:73 in my 70 4 speed and on the highway, I need to stay in the slow lane and let everything whiz by or quickly get the "R's" winding too high if I get into the gas for any length of time.

In the 69 at 55 mph, drop the automatic down a gear and nail it......... it was like launching at a rolling 55 MPH.

Something to be said for a highway gear and it's fun it it's own right.

This reminds me of my 327/powerglide 307 gear combo I had years ago. It didn't have much of a holeshot, but once it was going it was going and going, .....and going!

I have those "granny gears" in my '68 12 bolt w/ 396 and a monster cam...and I hate it off the line. 0 to 60 in about 5 minutes. I have been wanting to swap to a 3.55 or 3.73. If you want to sell me the set you pull out, drop me a line. I may be interested. :hurray:

I have 3.07 with a 402, it's not bad on the highway. I drive it to work every day. It will get off the line, too... you just need a big enough engine and enough rubber out back!

3.07 is a great highway gear,but not a good 1/4 mile...you'll cross the stripe at less than 4500 RPM
But if you don't race the car and just want to be able to cruise a long ride without being all wound up,go for it :thumbsup:

Our 70 conv has a 3.07 12bolt posi (from a 69). As mentioned, off the line isn't great, but it's really a street performance driver, not a hard core race car. With today's gas prices, I'm even considering going a 12bolt 2.73 that I acquired a few years ago.
The car has a healthy SB400 that I build and an M20 which handles the 3.07 just fine. If gas goes back to $.25/gal, I may be interested in installing a set of 4.10 gears, but until then, I'm staying with the 3.07! :thumbsup:

YouTube - CB08 Dragstrip pass

I have 3.08's in my 383 4 speed equiped 79 Camaro and they are fine. It's a great gear with a nice torquey engine and a 4 speed. I could see it being somewhat of a dog with an automatic though.

I could see it being somewhat of a dog with an automatic though.

Not when you have the torque of a 454 :D
Here is a 3.07 geared 69 SS that has an 87 octane burning,8 to 1 454 leaving the line:


http://www.chevelles.com/showroom/data/500/medium/bowman69SS.jpg

I stand corrected.

I know a guy that put 2:73 gears in his '68 for fuel economy (396 m20). He ended up putting a saginaw in it for a much friendlier low gear (3:50 I think) to make the car more driveable. He likes the car to drive now, but he doesn't abuse it either.

If my math is somewhat correct, changing from 3:55 to 3:07 will equivalent to changing my M20 which has a 2.52:1 ratio to a M21 transmission with 2.20:1.

At 1000rpm's with a M21 and 3.55 gears I get 128.04 revolution at the axel. And with a M20 and 3.07 gears I get 129.26 revolutions.

Don't race the car, just basicly drive to local cruise in shows. Looks like it would drop the rpms by about 15% when on the highway and probably increase the milage about the same. And my pinion seal leaks, so I need to replace that and probably the who pinion yoke, which is likley the problem.

Not when you have the torque of a 454 :D
Here is a 3.07 geared 69 SS that has an 87 octane burning,8 to 1 454 leaving the line:


http://www.chevelles.com/showroom/data/500/medium/bowman69SS.jpg


Hi Chris,

What is your best ET & MPH on street tires?

Hi Chris,

What is your best ET & MPH on street tires?

12.55 @ 108.01 MPH with 3.31 gears crossing the stripe @ 5000 RPM (motor makes max. HP @ 5700)

If my math is somewhat correct, changing from 3:55 to 3:07 will equivalent to changing my M20 which has a 2.52:1 ratio to a M21 transmission with 2.20:1..

Correct, BUT only for 1st, 2nd and 3rd gears... 4th will be 1:1 for either trans...



Don't race the car, just basicly drive to local cruise in shows. Looks like it would drop the rpms by about 15% when on the highway and probably increase the milage about the same. And my pinion seal leaks, so I need to replace that and probably the who pinion yoke, which is likley the problem.


Yes, not a huge difference in performance or mileage, but since you have the M20, it should still have decent performance...

I think I read that the M20 was std for the 3.31 and optional for the 3.07 (Mountain) and optional for the 3.55. And the M21 was standard with the 3.73 and optional with the 3.55 or 4.10 Thanks again everyone. Now I've got to deceide if I can do this or have a shop do it.