Gear ratios
Thread Starter
Intermediate
Joined: Oct 2013
Posts: 48
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From: Memphis TN
Gear ratios
Hello everyone,
I'm curious what the original gear ratio was in the 1968 corvette with a 427 and a 4-speed close ratio transmission.
I have read other threads and found suggestions on what is best to use but I haven't seen anything explaining what the original differential ratio was.
For context, I have a 68 with 4.11 rear end ratio. Originally, I had the 4-speed close ratio transmission but I have switched it out for a 6 speed from a Camaro. It's a great driving car but the exhaust has never fit right with the new transmission. It rattles and the heat transfer is worrisome. Now I'm considering putting the original trans back in the car so I can also get the original exhaust but a close ratio 4-speed with 4.11 rear end won't be very comfortable to drive.
I'm curious what the original gear ratio was in the 1968 corvette with a 427 and a 4-speed close ratio transmission.
I have read other threads and found suggestions on what is best to use but I haven't seen anything explaining what the original differential ratio was.
For context, I have a 68 with 4.11 rear end ratio. Originally, I had the 4-speed close ratio transmission but I have switched it out for a 6 speed from a Camaro. It's a great driving car but the exhaust has never fit right with the new transmission. It rattles and the heat transfer is worrisome. Now I'm considering putting the original trans back in the car so I can also get the original exhaust but a close ratio 4-speed with 4.11 rear end won't be very comfortable to drive.
Race Director
Joined: Mar 2017
Posts: 11,949
Likes: 4,507
From: Virginia
There were lots of them. If your code doesn't match what you've measured, perhaps someone swapped gears sometime in the past 53 years. It looks like everything except 2.73 was available for the 4-speed.
This site has lots of other code guides, but here's the one for axle ratios:
http://corvettec3.ca/axle.htm
You may find some additional information in the 1968 Corvette vehicle information kit from the GM Heritage Center Archives:
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
If a trans-tunnel-hugging exhaust doesn't fit, that's the universe's way of telling you to install side pipes.
This site has lots of other code guides, but here's the one for axle ratios:
http://corvettec3.ca/axle.htm
You may find some additional information in the 1968 Corvette vehicle information kit from the GM Heritage Center Archives:
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
If a trans-tunnel-hugging exhaust doesn't fit, that's the universe's way of telling you to install side pipes.
Last edited by Bikespace; May 1, 2021 at 09:01 AM.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
don't. you will hate it. more than an exhaust rattle. are you still running the original non-removable crossmember? that is the issue. there are many improvements on it. including homemade and homemod. i just made a new word! anyway, start researching crossmembers that will fit your trans and give you more room for the exhaust to pass through-over-under-whatever. you put that 4 speed back in, the first time you hit 55 mph you will try to shift into 5th.
Old Pro Solo Guy
Joined: Dec 2017
Posts: 8,088
Likes: 4,446
From: Marlton NJ
There will be a code on the bottom lip of the diff next to the spring. That code will tell you what the original gear was for your diff, if that matters to you.
If it doesn't matter run what you like.
My 72 454 had the std 3.36 with a CR from the factory. I could not stand it Too much clutch slipping, felt like it had airplane gears in 1st, 1st went to 60+ mph..
A 3.55 would be a slight but welcome improvement. But highway speeds quickly become a bother. My Z28 ran those and about 3500 at 70 mph. Not too comfortable or quiet in a BB. You could slow down I suppose.
So it depends on if you like around town driving, or want highway cruising.
With a CR box you have to pick one or the other. The box only has a 2.2 gear spread, it is not big enough to give you both.
A WR muncie box with a 3.36 or 3.55 would be a much better 1st gear combo, the 2.52 1st gear helps that much.
That gear spread improves to about 4.2 to 4.5 with the 5spd TKO I put in my car. The 6 spd even goes all the way to a 5.9 gear spread.
That's why the OD boxes work so well, you can have lively 1st gear, and OD cruising too. The second gear in my TKO will be the same overall as my 1st gear used to be!
BTW the 5spd TKO fits much better than the 6sp T56, which was your original issue.
Here is an Excel C3 trans & gear spreadsheet you can play with to your hearts content!
.
If it doesn't matter run what you like.
My 72 454 had the std 3.36 with a CR from the factory. I could not stand it Too much clutch slipping, felt like it had airplane gears in 1st, 1st went to 60+ mph..
A 3.55 would be a slight but welcome improvement. But highway speeds quickly become a bother. My Z28 ran those and about 3500 at 70 mph. Not too comfortable or quiet in a BB. You could slow down I suppose.
So it depends on if you like around town driving, or want highway cruising.
With a CR box you have to pick one or the other. The box only has a 2.2 gear spread, it is not big enough to give you both.
A WR muncie box with a 3.36 or 3.55 would be a much better 1st gear combo, the 2.52 1st gear helps that much.
That gear spread improves to about 4.2 to 4.5 with the 5spd TKO I put in my car. The 6 spd even goes all the way to a 5.9 gear spread.
That's why the OD boxes work so well, you can have lively 1st gear, and OD cruising too. The second gear in my TKO will be the same overall as my 1st gear used to be!
BTW the 5spd TKO fits much better than the 6sp T56, which was your original issue.
Here is an Excel C3 trans & gear spreadsheet you can play with to your hearts content!
.
Last edited by leigh1322; May 1, 2021 at 10:26 AM.
Melting Slicks
Joined: Jan 2006
Posts: 3,180
Likes: 297
From: Sarasota Florida
don't. you will hate it. more than an exhaust rattle. are you still running the original non-removable crossmember? that is the issue. there are many improvements on it. including homemade and homemod. i just made a new word! anyway, start researching crossmembers that will fit your trans and give you more room for the exhaust to pass through-over-under-whatever. you put that 4 speed back in, the first time you hit 55 mph you will try to shift into 5th.
