C3 Corvette Axle Codes and Gear Ratios 1968-1982
Last reviewed July 2026, maintained by Corvette-C3.com
topDecode your axle stamping
Type what is stamped on the differential, in any order. The decoder reads the ratio code, the build day, the plant, the positraction supplier and the shift. Where the stamping sits on the car, and what each element means ↓
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The axle code carries no model year, so you will usually need one of these too: C3 VIN decoder · trim tag decoder · RPO option codes
Decoding your rear axle code
Chevrolet stamped a short code into the differential housing itself, low on the case on a machined flat between the leaf spring mounting plate and the cover. It is a tight, grimy spot and the stamp usually needs cleaning before it will read. That code tells you the final drive ratio the car left the factory with, which decides how it pulls away and how hard it revs on the motorway, and which matters to anyone checking a car against its build sheet.
The tables further down list the ratio codes for every model year from 1968 to 1982. The stamping itself looks like this:
- OK the axle ratio. OK is 3.08:1, and only on a 1978 car
- 311 the day of the year it was built, so 7 November
- W the plant. W is Warren, Michigan, G is Detroit Gear and Axle
- 7E the positraction supplier. E is Eaton, D is Dana. The 7 is not a year
- 1 the shift that built it
One warning, because the internet gets this wrong. You will find diagrams claiming that a digit in front of the positraction letter gives the model year. It does not. This code carries no year at all, which is exactly why an axle built in November can sit in a car sold as the following model year. The ratio code does not settle it either. Chevrolet reused the letters: OK is 3.08:1 on a 1978 car and 2.72:1 on a 1981 one, and OA, OH, OJ and OF are reused the same way. To know which ratio your code means, you need the model year from the VIN or the trim tag. The decoder below lists every year a code appears in rather than guessing.
Two other things worth knowing. The three digit group is a day of the year rather than a date, so 311 means the 311th day, the 7th of November. And the format changed during the run: the early cars carry a shorter two letter ratio code with a four digit month and day, and the plant letter on the end. The letters you see below starting with O arrive in 1976.
Have a clear photograph of the stamping on your own car? Send it in through the contact page. The order of the middle elements is genuinely disputed and real photographs settle it faster than forum posts do.
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'.
1969 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: Most 1969 C3 Differentials were manufactured at Warren and are suffixed 'W'.
1969 models built after approximately August of 1969 use 1970 coded differentials.
1970 Corvette (Late 1969) Axle Ratios
| Code | Ratio | Application |
|---|---|---|
| CAK | 3.36 | Standard (350) |
| CAL | 3.08 | Standard (350) |
| CAM | 3.36 | Positraction (350) |
| CAN | 3.55 | Standard (350) |
| CAO | 3.70 | Positraction (350) |
| CAP | 4.11 | Standard (350) |
| CAS | 3.70 | Standard (350) |
| CAT | 3.08 | Heavy Duty Positraction (454) |
| CAU | 3.36 | Heavy Duty Positraction (454) |
| CAV | 3.08 | Standard (454) |
| CAW | 3.08 | Standard (454) |
| CAX | 3.36 | Heavy Duty Positraction (454) |
| CAY | 2.73 | Positraction (454 Turbo-Hydra-Matic) |
| CAZ | 3.55 | Heavy Duty Positraction (454) |
| CFA | 3.70 | Positraction (454) |
| CFB | 4.11 | Heavy Duty Positraction (454) |
| CFC | 4.56 | Heavy Duty Positraction (454) |
| CLR | 3.36 | Standard (454) |
Note: Positraction became standard equipment in 1970 and the reference of Standard axle codes is in conflict.
1971 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| AA | 3.55 |
| AB | 3.70 |
| AC | 4.11 |
| AD | 4.56 |
| AW | 3.08 |
| AX | 3.36 |
| LR | 3.36 |
1972 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| AA | 3.55 |
| AB | 3.70 |
| AC | 4.11 |
| AW | 3.08 |
| AX | 3.36 |
| LR | 3.36 |
1973 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| AA | 3.55 |
| AB | 3.70 |
| AC | 4.11 |
| AW | 3.08 |
| AX | 3.36 |
| LR | 3.36 |
1974 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| AA | 3.55 |
| AB | 3.70 |
| AC | 4.11 |
| AW | 3.08 |
| AX | 3.36 |
| LR | 3.36 |
1975 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| AA | 3.55 |
| AB | 3.70 |
| AC | 4.11 |
| AY | 2.73 |
| AW | 3.08 |
| AX | 3.36 |
| LR | 3.36 |
1976 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| OA | 3.08 |
| OD | 3.36 |
| LR | 3.36 |
| OB | 3.55 |
| OC | 3.70 |
1977 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| OA | 3.08 |
| OD | 3.36 |
| LR | 3.36 |
| OB | 3.55 |
| OC | 3.70 |
1978 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| OK | 3.08 |
| OM | 3.36 |
| OH | 3.55 |
| OJ | 3.70 |
1979 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| OM | 3.36 |
| OH | 3.55 |
| OJ | 3.70 |
1980 Corvette Axle Ratios
| Code | Ratio |
|---|---|
| OF | 3.07 |
| OH | 3.07 |
1981 Corvette Axle Ratios
| Code | Ratio | Application |
|---|---|---|
| OJ | 2.87 | Automatic Transmission |
| OK | 2.72 | Manual Transmission |
1982 Corvette Axle Ratios
| Code | Ratio | Application |
|---|---|---|
| OA | 2.72 | Standard Wheels |
| OF | 2.87 | Aluminum Wheels |
Frequently asked questions
Where is the axle code on a C3 Corvette?
Low on the differential housing itself, on a machined flat between the leaf spring mounting plate and the cover. It is a tight, grimy spot and the stamp usually needs cleaning before it will read. Once you have it, the free axle code decoder at the top of this page reads it for you.
Does a C3 Corvette axle code tell you the model year?
No. The stamping carries no year at all, which is exactly why an axle built in November can sit in a car sold as the following model year. Diagrams claiming that the digit in front of the positraction letter gives the year are wrong.
How do I find the gear ratio of my C3 Corvette?
The two or three letters at the start of the axle stamping give the final drive ratio. Chevrolet reused those letters, so OK is 3.08:1 on a 1978 car but 2.72:1 on a 1981 one. The decoder on this page lists every model year a code was used in rather than guessing one.