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.

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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.

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FYI – your Corvette must be equipped with a posi-traction differential for the above procedure to work.

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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.

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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

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23 thoughts to “Rear Axle/Rear End Ratio Information”

  1. I have a ’59 C1 Corvette that is being Resto-Mod’d with an LT1 w/ 8-speed Auto. I’ve heard of replacing the solid axle with a C4 rear-end. Do you have any info or suggestions on this topic?

  2. If I left this comment at the forum I likely get some arguments but a recent experience proves your point. I bought a 2009 Z06 with a 3.42 differential just six months ago. The drivetrain was guaranteed for a month and even though it was geared perfectly for the 505hp/480tq 427 I had to ask them to switch it out for a new one. It was grinding a little between every gear and I have been a 3 pedal man since I learned to drive a ’56 Chevy three-on-the-tree 45 years ago. When they removed the old transmission they informed me it was not stock but an aftermarket unit that was geared for a 2009 ZR1. My Z06 should have had a transmission with a 2.66 1st gear, instead it had a 2,29. Now, I am wondering if the replacement is a base unit with a 3.29 first because it had been so smooth and quick to drive when the gears did not grind. Now, it feels like someone put a transmission meant for a 4 cylinder in it. Its hard to keep up with traffic when leaving a stop light unless you rev it up to 3500rpm through the first two gears. I am glad I kept the aftermarket transmission. It will probably get repaired and put back in.

  3. I have a 1980 auto 350/350 not sure but think it is factory rear end. When I bought it, it had new p255 60r 15’s on it. My tag in door says it should have 225’s on it. The speedometer only goes to 80mph and I am almost hitting that while driving around 35 to 45 mph approx. It has a nineteen speed gear in trans. What would be the right gear for the speedometer in trans?

  4. Kinda weird performance in my C-6 2005 LT2. Hard left or sometimes right turns, usually slow but uphill, caused near stall effect when foot feed was reapplied. Currently car has 48,890 original miles. Took it to dealer and they determined that the two cat converters were plugged! Replaced them and now car runs without a problem. Like I noted, weird problem!

  5. i have a 2003 c5 corvette, its equipped with automatic-OD trans and 2,73 gear. currently I have rear tires sizes 285-35-18’s. presently adding engine upgrades, comp cam # 544411: 224-234 dur, 568-583 lift cold air, amer racing long tube headers w/magnaflow exh, 102mm throttle body. leaning towards 3.90 gears or 3.73 I’m interested in your perspective.

    1. It really comes down to your plan for the car. If you’re planning on 1/4 mile racing then you need to calculate based on a 1/4 mile gear ratio for your application based on the vehicle weight, vehicle horsepower, RPM through lights (finish line), and tire height.

  6. I have my eye on a low mileage (14,800 miles), one owner, 2011 C6, manual transmission. Before I make the purchase, I want to see what my budget will get me for a mild street rod. I am looking at the Lingenfelter heads package, good for about 525 hp, and the addition of a 3.73 rear end. Will the 3.73 rear end give me enough “jump” in gears 2-5, and still preserve comfortable cruising in 6th gear? Is a 4.10 or 4.11 rear end out of the question?

  7. I have a 2015 Z06 650hp and 6650’s torque. 8spd paddle shift. My window sticker says 3.42 ratio. My build sheet says 2.41 ratio and again the window sticker under options, 2LZ prefered equipment group says 2.73. What gears do i have? I have driven 15,540 miles with no problems with pick up or shifting. I run at about 2000rpm always since i usually am in sport or track mode.

    1. All c7 Z06 autos have a 2.42 rear end ratio. The first gear is a 4.56 in the 8 speed auto, so overall 1st gear ratio is 11.035:1

  8. How much horsepower increase would a power chip suited for a 1987 Corvette engine give me? It is rated at 240 hp. How high would it go up?

  9. I have a 1987 Vette conv., and I have wondered how I can increase performance without tearing deep into the engine. It has the automatic tranny with, I believe, the 3.07 rear axle. Any suggestions?

      1. I have heard that power chips don’t really offer much improvement in performance. They may work to some degree but it’s hardly noticeable. What’s your position?

  10. I have a stock l 82 4 speed with very low gear ratios so 60 MPhil feels like I am pushing it. What should I do to get better milage and higher gear ratio. Thanks chuck

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