1953-1982 Rear Axle Ratios
Rear Axle/Rear End Ratio Information
Have you ever thought about how the correct rear axle ratio affects your Corvette’s performance? First, learn what ratio you have hidden away in the differential housing. Early Corvettes (1960-1970) may have many different ratios; later Corvettes had fewer choices, in an effort to boost fuel mileage, and by the mid-80s performance gear ratio options were rare.
Raise the rear of your Corvette so both wheels spin freely. You need a jack and stands, or a lift if you are lucky. Be extra careful when lifting any vehicle. If using a floor jack, make sure the front wheels roll as the jack lifts the back end. Place wheel chocks at the front tires to prevent rolling forward. As the jack lifts, your Corvette has to roll back towards you as the distance becomes shorter.
Once in the air, stands should be placed under the frame or trailing arms as protection from jack failure. When your Corvette is securely lowered, rear axle ratio can be safely determined. The transmission must be in neutral with the parking brake off. The objective is to count how many times the driveshaft rotates as the wheel makes one full rotation. Mark the driveshaft near the differential housing (usually the yoke) and make a mark on the differential housing. Mark the tire so you can determine when one full rotation has taken place.
FYI – your Corvette must be equipped with a posi-traction differential for the above procedure to work.
Early Corvettes without posi-traction will require a slightly different approach; only the wheel should be up off the ground, rotating freely. Use the same procedure as above to safely raise and support your Corvette. The free spinning wheel has to be rotated two full turns while counting how many times the driveshaft rotates. This is due to the differential pinion gears rotating inside the differential. You can use the same procedure as the posi-traction equipped Corvette although it is difficult and an error can easily occur. You would require a helper and both wheels must rotate in unison while someone counts the driveshaft rotations.
The equation is simple. If the driveshaft yoke rotates a bit past four turns during one full wheel rotation, you would have a 4.11 rear gear ratio. Divide the driveshaft rotations into one [4.11 / 1]. Non posi-traction Corvettes use the same formula, counting two revolutions of the wheel while using one as the divisor.
What do the numbers mean?
Numerically low gear ratio numbers (3.08, for example) mean more power/torque is required to move your Corvette. Once rolling, the 3.08 gear ratio will keep your engine rpm low and help save fuel. On the flip side, starting line performance suffers. High numerical gear ratios (4.11) have a greater mechanical advantage, requiring less power (torque) to get you rolling. The downside is engine rpm increases at highway speeds as fuel flows rapidly out of your tank.
Wheel/tire diameter also plays an important part in the rear gear ratio equation. Installing a taller tire lowers the mechanical advantage, requiring less rpm to maintain the same speed as the original equipment tire diameter. This affects acceleration performance negatively while increasing top end speed.
Do I Really Care if I Have a Low or High Gear Ratio?
Yes, if overall performance is important to you and especially if you’re planning any performance modifications. The most powerful engine will feel sluggish with the wrong rear axle gear ratio. In some cases, the rear axle gear ratio should be left alone and a transmission change may be the correct direction to take. Some contrary situations arise when dealing with mechanical components, including the driveline. Let me expand on what driveline situations you might be up against.
Overdrive Transmission or Rear Axle Gear Ratio Change?
Overdrive transmission is a great innovation. Early on, overdrive transmissions were marketed with poor results. No one was concerned with fuel mileage. Today, overdrive transmission is considered standard equipment. However, many early Corvette restorers are reluctant to make this modification. Not only do you slow the engine down at highway speed, many overdrive units have a better first gear ratio for more starting line performance.
This means you also need to consider the transmission’s gear ratios for best possible driveline performance. In some cases, changing the transmission to an overdrive unit can help off-the-line performance and save fuel while cruising. Where am I going with this? Take time to analyze what you have to work with. You may be surprised to find not much engine work is required to gain the desired performance.
I know there is never enough performance, but this will send you down the right path to get all you can.
Compound/Combined Gearing
Let’s review how the rear axle gear ratio works in conjunction with the transmission. All Corvettes built before 1981 had 1-1 transmission final drive gear ratios. This is the prime example of how an overdrive transmission change would benefit: 1969 Corvette with an original Muncie M20 that has a 2.52 first gear ratio with a 3.36 rear axle gear ratio. Our combined first gear ratio would be [3.36 x 2.52 = 8.46], not too bad for a 427 cubic inch engine with plenty of low rpm torque. A low torque 327 cubic inch engine would not be very exciting to drive until you got above 35-40 mph.
Changing to a Tremec TKO 500 manual transmission—which has a 3.27 first gear ratio and .68 overdrive ratio—would make this the perfect for performance and cruising. The 3.36 rear axle gear ratio coupled with the 3.27 transmission first gear ratio multiplies providing a 10.98 first gear ratio [3.36 x 3.27 = 10.98]. That 10.98 combined gear ratio will make even a smog motor small block Chevy feel good.
If you went the other way and changed the rear axle ratio, the engine would scream—4000 rpm at 75 mph with a 4.11 rear gear axle ratio, adding extra wear to the engine and to your mental state. The combined gear ratios [4.11 x 2.52 = 10.35] would provide decent off-the-line performance, even with a low horsepower engine. This is why it makes sense to evaluate what you are working with before making any driveline changes, including engine performance.
Tremec also has a TKO 600 five speed overdrive transmission available with a 2.87 first gear ratio. This might be used in engines that produce high torque at low rpm (big blocks or large cubic inch small blocks). Using a 3.90 rear axle gear ratio as an example [2.87 x 3.90 = 11.19], this is on the high side for a high torque engine. A 3.36 rear axle gear ratio would be more livable [3.36 x 2.87 = 9.64]. Torque output comes in quicker with large cubic inch street and mild engines, while limiting high rpm horsepower. Now there are large cubic inch race engines that make good torque and horsepower at higher numbers. That is another article in itself.
Choosing a TKO 500 five speed overdrive transmission for a large cubic engine would require rear axle gear ratios in the high-mid twos. The TKOs 3.27 first gear [3.27 x 2.73= 8.92] makes good use of a large cubic inch engine’s torque. If a 3.50 rear axle gear ratio was used, first gear starts would be very short lived; great for a heavy duty truck with a 10,000 pound gross vehicle weight.
Automatic Transmission Torque Converters also play into the Equation
Torque converters are more than a slipping clutch to stop your Corvette at the light without taking the transmission out of gear. Torque converters multiply torque to move the vehicle’s weight. Typical torque conversion is a 2:1 multiplication rate. If your engine is producing 250 foot pounds of torque, it would be in the 500 foot pound range as the torque converter approaches stall speed. That means the same rear axle gear ratio with an automatic transmission would help you leave the stoplight quicker.
As far as automatic transmission first gear ratios are concerned, the list below covers all factory equipped automatics. 4L60 and 4L60E have .70 overdrive gear ratio. The same formulas apply for the automatic gear ratios concerning the combined ratio. The beauty is in the converter’s torque multiplication.
TH350 2.52
TH400 2.48
700R4 or 4L60 3.06
4L60E 3.06
An automatic Corvette could potentially have a better launch than a manual. Automatic transmissions are also easier on the drivetrain, avoiding the shock loading when the clutch is released quickly on manual transmissions.
You can expect improved performance from a rear axle gear ratio change on 1982 and up Corvettes equipped with overdrive transmissions.
When fuel mileage standards were raised, auto manufacturers decided an overdrive transmission was the easiest way to comply. In an attempt to increase the fuel mileage even further, high mechanical advantage rear axle ratios were used. It was common to find 3.08 and lower rear axle gear ratios in 1982 and older vehicles. 1984 Corvette Crossfires took full advantage of this technology with 3.07-3.31 rear axle gear ratios and an engine that ran out of power at 4000 rpm due to the restrictive intake manifold.
By 1985-1991, the L98 Tuned Port Injection was introduced with excellent torque at low engine rpm, although horsepower faded above 4500 rpm. This required a conservative rear axle gear ratio choice. Later, when LT1, LT4, and LS series engines were introduced, engine torque and horsepower levels increased. There was no longer a need for 2.50 or 2.73 rear axle gear ratios to take advantage of the L98 Tuned Port engine’s torque.
A great example is an early 90s L98 engine with 2.59 rear axle ratio. Changing to a 3.50 rear axle gear ratio makes them feel like they gained 50 horsepower. Now the same 90s Corvette does not do as well with a 4.11 rear axle gear ratio, unless you plan to run full throttle stoplight to stoplight. This is how gear ratios make a direct impact on vehicle driveline performance.
Let’s Wrap it Up
Now that we know how driveline ratios play into overall performance, let’s cover how important it is to match engine torque curve to gear ratios. Camshaft specification predicates torque curve rpm band. A typical early Corvette engine has torque/horsepower band in the 2000-5500 range. To sum things up, large cubic inch engines tend to have more torque at lower rpm, performing better with lower combined gear ratios. Small cubic engines require more rpm before torque builds, requiring higher combined gear ratios.
All too often, I see owners who are disappointed with poor performance after major high performance engine work has been completed. Most of the time, too much camshaft duration and incorrect combination of gear ratios are to blame.
Always consider what type of driving you expect to do. If you’re drag racing, the rear axle ratio should be 3.90 or higher. Everyday cruiser rear axle gear ratios tend to be less aggressive, in the 3.50 or lower range. Time to do the calculations!
1953-1967 Rear Axle Ratios & Identification Codes
| Code | Ratio | Type | Application |
|---|---|---|---|
| 1953 | |||
| LW | 3.55:1 | Standard | Early |
| MW | 3.55:1 | Standard | Late |
| 1954 | |||
| MW | 3.55:1 | Standard | All |
| 1955 | |||
| AE | 3.55:1 | Standard | Powerglide |
| AH | 3.55:1 | Standard | 3-Speed |
| AD | 3.27:1* | Standard | 3-Speed |
| *May not have been used on the 1955 Corvette | |||
| 1956 | |||
| AE | 3.55:1 | Standard | Powerglide |
| AH | 3.70:1 | Standard | 3-Speed |
| AD | 3.27:1 | Standard | 3-Speed |
| AJ | 4.11:1 | Standard | 3-Speed |
| NOTE: A very few 1956 Corvettes used Positration Rear Axles stamped “3.70.11 HT” or “4.56.1HT” indicating gear ratio and Hi-Tork. | |||
| 1957-1958 | |||
| AE | 3.36:1 | Standard | Powerglide |
| AH | 3.70:1 | Standard | Manual Transmission |
| AN | 3.70:1 | Positraction | Manual Transmission |
| AP | 4.11:1 | Positraction | Manual Transmission |
| AQ | 4.56:1 | Positraction | Manual Transmission |
| AS | 3.70:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| AT | 4.11:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| AU | 4.56:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| 1959-1960 | |||
| AE | 3.55:1 | Standard | Powerglide |
| AH | 3.70:1 | Standard | Manual Transmission |
| AN | 3.70:1 | Positraction | Manual Transmission |
| AP | 4.11:1 | Positraction | Manual Transmission |
| AQ | 4.56:1 | Positraction | Manual Transmission |
| AS | 3.70:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| AT | 4.11:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| AU | 4.56:1 | Positraction | Manual Trans. & H.D. Brakes & Suspension |
| FJ | 3.70:1 | Standard | Manual Trans. & Metallic Brakes |
| FK | 3.70:1 | Positraction | Manual Trans. & Metallic Brakes |
| FL | 4.11:1 | Positraction | Manual Trans. & Metallic Brakes |
| FM | 4.56:1 | Positraction | Manual Trans. & Metallic Brakes |
| 1961 | |||
| AC | 3.36:1 | Standard | 3-Speed |
| AE | 3.55:1 | Standard | Powerglide |
| AH | 3.70:1 | Standard | 4-Speed |
| AN | 3.70:1 | Positraction | Manual Transmission |
| AP | 4.11:1 | Positraction | Manual Transmission |
| AQ | 4.56:1 | Positraction | Manual Transmission |
| AF | 3.36:1 | Positraction | Manual Transmission |
| AS | 3.70:1 | Positraction | 4-Speed & H. D. Brakes & Suspension |
| AT | 4.11:1 | Positraction | Manual Trans. & H. D. Brakes & Suspension |
| AU | 4.56:1 | Positraction | Manual Trans. & H. D. Brakes & Suspension |
| FJ | 3.70:1 | Standard | 4-Speed & Metallic Brakes |
| FK | 3.70:1 | Positraction | 4-Speed & Metallic Brakes |
| FL | 4.11:1 | Positraction | Manual Trans. & Metallic Brakes |
| FM | 4.56:1 | Positraction | Manual Trans. & Metallic Brakes |
| 1962 | |||
| CA | 3.36:1 | Standard | Manual Transmission or Powerglide |
| CB | 3.36:1 | Positraction | Manual Transmission or Powerglide |
| CC | 3.55:1 | Positraction | 4-Speed |
| CD | 3.70:1 | Positraction | 4-Speed |
| CE | 4.11:1 | Positraction | 4-Speed |
| CF | 4.56:1 | Positraction | 4-Speed |
| CG | 3.70:1 | Standard | 4-Speed |
| CX* | 3.70:1 | Standard | Close-Ratio 4-Speed |
| CH | 3.36:1 | Standard | Manual Trans. & Metallic Brakes |
| CY | 3.70:1 | Standard | 4-Speed & Metallic Brakes |
| CK | 3.36:1 | Positraction | 4-Speed & Metallic Brakes |
| CL | 3.55:1 | Positraction | 4-Speed & Metallic Brakes |
| CM | 3.40:1 | Positraction | 4-Speed & Metallic Brakes |
| CN | 4.11:1 | Positraction | 4-Speed & Metallic Brakes |
| CP | 4.56:1 | Positraction | 4-Speed & Metallic Brakes |
| CQ | 3.70:1 | Positraction | 4-Speed & H. D. Brakes & Suspension |
| CR | 4.11:1 | Positraction | 4-Speed & H.D. Brakes & Suspension |
| CS | 4.56:1 | Positraction | 4-Speed & H.D. Brakes & Suspension |
| CT | 3.08:1 | Standard | 4-Speed |
| CU | 3.08:1 | Positraction | 4-Speed |
| CV | 3.08:1 | Standard | 4-Speed & Metallic Brakes |
| CW | 3.08:1 | Positraction | 4-Speed & Metallic Brakes |
| *1962 “CX” Code Unverified | |||
| 1963-1964 | |||
| CA | 3.36:1 | Standard | Manual Transmission or Powerglide |
| CB | 3.36:1 | Positraction | Manual Transmission or Powerglide |
| CC | 3.55:1 | Positraction | 4-Speed |
| CD | 3.70:1 | Positraction | 4-Speed |
| CE | 4.11:1 | Positraction | 4-Speed |
| CF | 4.56:1 | Positraction | 4-Speed |
| CJ | 3.08:1 | Positraction | 4-Speed |
| CX | 3.70:1 | Standard | Close-Ratio 4-Speed |
| CZ | 3.08:1 | Standard | Wide-Ratio 4-Speed |
| 1965-1967 | |||
| AK | 3.36:1 | Standard | Manual Transmission or Powerglide (327) |
| AL | 3.08:1 | Positraction | 4-Speed (327) |
| AM | 3.36:1 | Positraction | Manual Transmission (327) |
| AN | 3.55:1 | Positraction | 4-Speed (327) |
| AO | 3.70:1 | Positraction | 4-Speed (327) |
| AP | 4.11:1 | Positraction | 4-Speed (327) |
| AQ* | 4.56:1 | Positraction | 4-Speed (327) |
| AR* | 3.08:1 | Standard | Wide-Ratio 4-Speed (327) |
| AS | 3.70:1 | Standard | Close-Ratio 4-Speed (327) |
| AT | 3.08:1 | Positraction | 4-Speed (396/427) |
| AU | 3.36:1 | Positraction | 4-Speed (396/427) |
| AZ | 3.55:1 | Positraction | 4-Speed (396/427) |
| FA | 3.70:1 | Positraction | 4-Speed (396/427) |
| FB | 4.11:1 | Positraction | 4-Speed (396/427) |
| FC | 4.56:1 | Positraction | 4-Speed (396/427) |
| NOTE: All 1965-167 Corvette Rear Axles were manufactured at Warren and are suffixed “W”. | |||
| *Listed 1967 “AQ” and “AR” codes & ratios may only have been used early in the 1967 model-year, or not at all. | |||
| 1968-1969 | |||
| AK | 3.36:1 | Standard (327/350) | |
| AL | 3.08:1 | Positraction (327/350) | |
| AM | 3.36:1 | Positraction (327/350) | |
| AN | 3.55:1 | Positraction (327/350) | |
| AO | 3.70:1 | Positraction (327/350) | |
| AP | 4.11:1 | Positraction (327/350) | |
| AS | 3.70:1 | Standard (327/350) | |
| AT | 3.08:1 | Heavy Duty Positraction (427) | |
| AU | 3.36:1 | Heavy Duty Positraction (427) | |
| AV | 3.08:1 | Positraction (427) | |
| AW | 3.08:1 | Heavy Duty Positraction (427) | |
| AY | 2.73:1 | Heavy Duty Positraction (427 Turbo Hydra-Matic) | |
| AZ | 3.55:1 | Heavy Duty Positraction (427) | |
| FA | 3.70:1 | Heavy Duty Positraction (427) | |
| FB | 4.11:1 | Heavy Duty Positraction (427) | |
| FC | 4.56:1 | Heavy Duty Positraction (427) | |
| NOTE: 1968-69 Corvette Rear Axles were manufactured at Warren and are suffixed “W”. 1969 models built after approximately August of 1969 use 1970 coded rear axles. | |||
| 1970 (Late 1969) | |||
| CAK | 3.36:1 | Standard | |
| CAL | 3.08:1 | Standard | |
| CAM | 3.36:1 | Positraction | |
| CAN | 3.55:1 | Standard | |
| CAO | 3.70:1 | Positraction | |
| CAP | 4.11:1 | Standard | |
| CAS | 3.70:1 | Standard (LT1) | |
| CAT | 3.08:1 | Heavy Duty Positraction | |
| CAU | 3.36:1 | Heavy Duty Positraction | |
| CAV | 3.08:1 | Standard | |
| CAW | 3.08:1 | Standard | |
| CAX | 3.36:1 | Heavy Duty Positraction | |
| CAY | 2.73:1 | Positraction (454 Turbo Hydra-Matic) | |
| CAZ | 3.55:1 | Heavy Duty Positraction | |
| CFA | 3.70:1 | Positraction | |
| CFB | 4.11:1 | Heavy Duty Positraction | |
| CFC | 4.56:1 | Heavy Duty Positraction | |
| CLR | 3.36:1 | Standard | |
| NOTE: Positraction became standard equipment on the 1970 Corvette and the inclusion of Non-Positraction (standard) axle codes by Chevrolet is in conflict | |||
| 1971 | |||
| AA | 3.55:1 | ||
| AB | 3.70:1 | ||
| AC | 4.11:1 | ||
| AD | 4.56:1 | ||
| AW | 3.08:1 | ||
| AX | 3.36:1 | ||
| LR | 3.36:1 | ||
| 1972 | |||
| AA | 3.55:1 | ||
| AB | 3.70:1 | ||
| AC | 4.11:1 | ||
| AX | 3.36:1 | ||
| LR | 3.36:1 | ||
| 1973-1974 | |||
| AA | 3.55:1 | ||
| AB | 3.70:1 | ||
| AC | 4.11:1 | ||
| AW | 3.08:1 | ||
| AX | 3.36:1 | ||
| LR | 3.36:1 | ||
| 1975 | |||
| AA | 3.55:1 | ||
| AB | 3.70:1 | ||
| AC | 4.11:1 | ||
| AY | 2.73:1 | ||
| AW | 3.08:1 | ||
| AX | 3.36:1 | ||
| LR | 3.36:1 | ||
| 1976-1977 | |||
| OA | 3.08:1 | ||
| OD | 3.36:1 | ||
| LR | 3.36:1 | ||
| OB | 3.55:1 | ||
| OC | 3.70:1 | ||
| 1978 | |||
| OK | 3.08:1 | ||
| OM | 3.36:1 | ||
| OH | 3.55:1 | ||
| OJ | 3.70:1 | ||
| 1979 | |||
| OM | 3.36:1 | ||
| OH | 3.55:1 | ||
| OJ | 3.70:1 | ||
| 1980 | |||
| OF | 3.07:1 | ||
| OH | 3.07:1 | ||
| 1981 | |||
| OJ | 2.87:1 | Automatic Transmission | |
| OK | 2.72:1 | Manual Transmission | |
| 1982 | |||
| OA | 2.72:1 | Standard Wheels | |
| OF | 2.87:1 | Aluminum Wheels | |
| NOTE: All 1970-1982 Corvette Rear Axles were Positraction | |||
| C4 | |||
| GHO | 3.54 | ||
| GM1 | 2.59 | Rear Axle | |
| GM3 | 3.45 | Rear Axle | |
| GT7 | 3.33 | Rear Axle | |
| GU2 | 2.73 | Rear Axle | |
| GW4 | 3.31 | Rear Axle | |
| HE3 | 3.07 (1984) | Rear Axle | |
| G44 | 3.07 (1985) | Rear Axle | |
| GUU | 3.07 (1986-1996) | Rear Axle | |
Story and photos courtesy Chris Petris
