Rear end ratio ?
Thread Starter
Instructor
Joined: Jun 2009
Posts: 157
Likes: 1
From: Sebring florida
Rear end ratio ?
Not having any paper work or tank sticker, how does one determine the rear end ratio.
I've been lerking around for a while and this forum is the best.
With the input from Dub I got my clutch pedal even with the brake pedal. Likewise with the help of the stickey's I got my brakes blead properly and timing set right.
Thanks for the help
Flyboy
I've been lerking around for a while and this forum is the best.
With the input from Dub I got my clutch pedal even with the brake pedal. Likewise with the help of the stickey's I got my brakes blead properly and timing set right.
Thanks for the help
Flyboy
Instructor
Joined: Nov 2008
Posts: 157
Likes: 0
I've got a spreadsheet that you can use if you want it, not sure how to send it to you unless you want to send me a message with your email. It has gear ratios for an M20, M21, M22, M22W and 2 TKO 5 speeds. You set whatever rear end ratio you want and specify tire diameter and it calculates speed and rpm in each gear.
To answer your question you can tell what rear gear you have by your tachometer and knowing how fast your going. For example, I'm pretty sure my car is a 3.08 rear and that my speedo has a gear for a 3.36. I can tell this because my car indicated 70mph in 4th @ 3000 rpm, but I'm passing and keeping up with traffic that is going faster than 70mph on the freeway. Also went past a radar traffic sign and was about 7mph off. The problem arises that you need to know your speed, which in my case would have thrown me off if I just went by the speedo. If yours reads correct here are RPM in 4th (1:1) with 27" tires at 70 mph for:
3.08 = 2684 RPM
3.36 = 2928 RPM
3.55 = 3094 RPM
3.73 = 3251 RPM
To answer your question you can tell what rear gear you have by your tachometer and knowing how fast your going. For example, I'm pretty sure my car is a 3.08 rear and that my speedo has a gear for a 3.36. I can tell this because my car indicated 70mph in 4th @ 3000 rpm, but I'm passing and keeping up with traffic that is going faster than 70mph on the freeway. Also went past a radar traffic sign and was about 7mph off. The problem arises that you need to know your speed, which in my case would have thrown me off if I just went by the speedo. If yours reads correct here are RPM in 4th (1:1) with 27" tires at 70 mph for:
3.08 = 2684 RPM
3.36 = 2928 RPM
3.55 = 3094 RPM
3.73 = 3251 RPM
Le Mans Master
Joined: Aug 2006
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
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Melting Slicks
Joined: Mar 2008
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From: North Bellmore Ny
St. Jude Donor '09
Pro
Joined: Feb 2002
Posts: 709
Likes: 10
From: Carlsbad CA
on the bottom of the differential case there is a code stamped in it. You can then look up what ratio was installed. If you still have the original gears that is all you need.
Rear Axle Identification
Used 1972-1975
Ratio Code
3.36 AX
3.36 LR
4.11 AC
3.70 AB
3.55 AA
3.08 AW
2.73 AV
Rear Axle Identification
Used 1972-1975
Ratio Code
3.36 AX
3.36 LR
4.11 AC
3.70 AB
3.55 AA
3.08 AW
2.73 AV
Last edited by hdpete98; Jun 23, 2009 at 11:46 PM.
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Drifting
Joined: Jul 2002
Posts: 1,697
Likes: 3
From: Woodinville WA
The codes will tell you what was originally inside the differential, but that may no longer be what is really inside.
1968 Corvette Axle Ratios
Code Ratio Application
AK 3.36 Standard (327/350)
AL 3.08 Positraction (327/350)
AM 3.36 Positraction (327/350)
AN 3.55 Positraction (327/350)
AO 3.70 Positraction (327/350)
AP 4.11 Positraction (327/350)
AS 3.70 Standard (327/350)
AT 3.08 Heavy Duty Positraction (427)
AU 3.36 Heavy Duty Positraction (427)
AV 3.08 Positraction (427)
AW 3.08 Heavy Duty Positraction (427)
AY 2.73 Heavy Duty Positraction (427 Turbo-Hydra-Matic)
AZ 3.55 Heavy Duty Positraction (427)
FA 3.70 Heavy Duty Positraction (427)
FB 4.11 Heavy Duty Positraction (427)
FC 4.56 Heavy Duty Positraction (427)
Note: 1968 C3 Differentials were manufactured at Warren and are suffixed 'W'.
1968 Corvette Axle Ratios
Code Ratio Application
AK 3.36 Standard (327/350)
AL 3.08 Positraction (327/350)
AM 3.36 Positraction (327/350)
AN 3.55 Positraction (327/350)
AO 3.70 Positraction (327/350)
AP 4.11 Positraction (327/350)
AS 3.70 Standard (327/350)
AT 3.08 Heavy Duty Positraction (427)
AU 3.36 Heavy Duty Positraction (427)
AV 3.08 Positraction (427)
AW 3.08 Heavy Duty Positraction (427)
AY 2.73 Heavy Duty Positraction (427 Turbo-Hydra-Matic)
AZ 3.55 Heavy Duty Positraction (427)
FA 3.70 Heavy Duty Positraction (427)
FB 4.11 Heavy Duty Positraction (427)
FC 4.56 Heavy Duty Positraction (427)
Note: 1968 C3 Differentials were manufactured at Warren and are suffixed 'W'.
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
I've got a spreadsheet that you can use if you want it, not sure how to send it to you unless you want to send me a message with your email. It has gear ratios for an M20, M21, M22, M22W and 2 TKO 5 speeds. You set whatever rear end ratio you want and specify tire diameter and it calculates speed and rpm in each gear.
To answer your question you can tell what rear gear you have by your tachometer and knowing how fast your going. For example, I'm pretty sure my car is a 3.08 rear and that my speedo has a gear for a 3.36. I can tell this because my car indicated 70mph in 4th @ 3000 rpm, but I'm passing and keeping up with traffic that is going faster than 70mph on the freeway. Also went past a radar traffic sign and was about 7mph off. The problem arises that you need to know your speed, which in my case would have thrown me off if I just went by the speedo. If yours reads correct here are RPM in 4th (1:1) with 27" tires at 70 mph for:
3.08 = 2684 RPM
3.36 = 2928 RPM
3.55 = 3094 RPM
3.73 = 3251 RPM
To answer your question you can tell what rear gear you have by your tachometer and knowing how fast your going. For example, I'm pretty sure my car is a 3.08 rear and that my speedo has a gear for a 3.36. I can tell this because my car indicated 70mph in 4th @ 3000 rpm, but I'm passing and keeping up with traffic that is going faster than 70mph on the freeway. Also went past a radar traffic sign and was about 7mph off. The problem arises that you need to know your speed, which in my case would have thrown me off if I just went by the speedo. If yours reads correct here are RPM in 4th (1:1) with 27" tires at 70 mph for:
3.08 = 2684 RPM
3.36 = 2928 RPM
3.55 = 3094 RPM
3.73 = 3251 RPM
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
if your speedometer is reading lower than your actual speed then the speedo cable is spinning too slow, speed it up by going one size smaller on your speedometer driven gear. easy to replace, just pull it out at the speedo connection at your tranny..
vice versa if your speedometer is reading too fast...
The replacements are color coded... try national parts depot or your dealer...
www.npdlink.com
click on the camaro book and search , they are all the same..
vice versa if your speedometer is reading too fast...
The replacements are color coded... try national parts depot or your dealer...
www.npdlink.com
click on the camaro book and search , they are all the same..
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I.D. codes on the housing are nice...but you probably don't know if anyone has changed the gears sometime in the past. Count the driveshaft revs (estimate a fraction of the last rotation if necessary) for one full turn of the rear wheels.
