Rear End Differentials: Gear Ratios, Controlled Differentials and Speedometer Gears

[Jason Edge]

adillac offered an optional "controlled differential". This is also known as a limited slip/ or posi-traction rear end.  Unlike a regular rear end, that directs power to the wheel with least traction, a controlled/posi rear end directs power to the wheel with most traction...meaning better traction and stability.

As far as the gear ratios offered here is a list by body style and any options.  The ID Number is stamped on the front face of the carrier assembly.   See picture at bottom with blue arrow pointing to ID Stamp.

ID Number        Body Style/Option     Gear Ratio

2                       reg. body style/non AC  2.94 - 1

1                        reg. body style/ AC        3.21  - 1(optional on non-AC reg. body styles)

6                        75 series body style       3.36 - 1

7                        Commercial Chassis      3.77  - 1(optional on 75 Series body styles)
Posi/Limited Slip was optional and was denoted by a G in front of the ID Number on the front face of the carrier.   You can also determine a limited slip/posi traction by blocking the front wheels, and lifting the entire rear end up in the middle at the differential.  Spin one of the tires. If both spin in the same direction it is a controlled/posi rear end. If they do not both spin in same direction or spin in opposite direction it is  a regular rear end.  

Other tell-tell signs is the special controlled differential label on the under side of the trunk lid and there is usually a metal ring around the differential filler plug that indicates it is a controlled differential and uses special lubricant.

Different Speedometer Gears used:

-57 to 64 Hydramatic (75 series), part # 351 1353, axle ratio 3.36, 21 teeth
-57 to 64 Hydramatic (75 series & 62 to 64 CC), part # 351 1354, axle ratio 3.77, 23 teeth
-59 to 63 Hydramatic (except 75 series, CC) and 1964 62 series, part # 351 1629, 2.94, axle ratio 2.94, 18 teeth
-59 to 63 Hydramatic (except 75 series, CC) and 1964 62 series,  part # 351 1630, axle ratio 3.21, 20 teeth
-64 Turbo Hydramatic , part # A148 1859 axle ratio 3.21, 40 teeth
-64 Turbo Hydramatic, part # B135 9271, axle ratio 2.94, 37 teeth
-64 Turbo Hydramatic, part # A136 2049, axle ratio 3.77, 42 teeth

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Location of Rear End ID Tag. With the "1" it is a 3.21 rear end.
If it was preceded by a "G" it would indicate a controlled (limited slip/Posi) rear end.  Here is a picture of one with the G1 Stamped:

 

 

[Jason Edge]

Very Nice! Nothing like freshly painted parts going back on the car.

[Jason Edge]

Casting #'s often do no match Cadillac Parts #, especially for items that had multiple uses, or used in different setups. For example the casting # of the 1964 to 1967 Cylinder head is  1481395... one cylinder head casting for both sides; however the MPB RH part # is 363 2376, and LH part # is 363 2375.
There are literally thousands of examples of Casting #'s not matching part #'s. I would start simple and ask for the location of their casting #, or better yet send a picture, and look and see what casting # you have on your differential.  If it the same, you are in luck. If it is different, it still could be for the same application, as I have seen casting #'s change even for the same part #!

[Jason Edge]

I believe I have both a 2.94 and 3.21 differential assemblies in storage, both are regular hypoid type rear ends; and a 3.21 limited slip Posi rear end coming out of current parts car, but have several on my Wait List for that Hot Item.

[Jason Edge]

It takes a lot of torque and power to move these 5000 lb beasts from a stand still to cruising speed.  The AC option puts a power hit on the engine, and adds a bit of weight to the car.  By lowering the gear ratio (higher gear #), you can pull the car easier off the line. Think of it as starting up a hill on a 10 speed bicycle in say 5th gear vs 2nd gear. You might not have much top end speed if you stay in 2nd gear, but you can surely get moving up that hill with much less effort.  The draw back is the engine is reving faster at highway speeds and loose a bit of the top end power/torque. Drag racers that want to expend all their power in the 1st 1/4 or 1/8 mile will run a very low gear. 
With all that said, regardless if you have AC or  not, if I had an option o go with the Higher 2.94 gear, or the Lower 3.21 gear, I would consider how the car is being driven.  If your car is spending most of its time highway cruising and traveling the higher gear makes more sense and will save you on gas mileage. If on the other hand, you are typically doing short, stop and go city driving the lower 3.21 gear will be easier on the drive train, get you up to city speed limit faster, and save on gas.  .... So as they say... it depends.

[David Thomas]

Hi Anders. I have a 63 A/C car that had the factory 3.21 rear differential. I am running 225/75R 15 tires on it. Those tires are about an inch or more smaller in diameter than the 8.20 15 tires that came on the car. That threw the speedometer off about 10 percent. Every 10 miles added an extra mile to the odometer. I found a good 2.94 differential and swapped them out, that ratio difference corrected the speedometer speed indication and odometer. I did notice a reduction in off the line performance after doing that, but the engine speed was reduced at highway speeds. I never have tried to be the first one out of the hole at a stop light, so that really didn't make much difference to me.

Here is a formula you can use to determine your drive shaft rpm at any given speed. If you know the final drive ratio, you can calculate the engine speed also. On the 63, it's easy, because the final drive in the hydra-matic is 1:1.

MPH X 336 X Differential Ratio divided by Tire Height = Drive Shaft RPM.

Here is my cars numbers at 60 MPH.

60 X 336 = 20160 X 2.94 = 59270.4 / 28.3" = 2094.36 D/S RPM, which equals the engine RPM on a 63 with 1:1 final transmission drive.

[Jason Edge]

Added a couple of pictures showing a differential with the G stamped into the face of the carrier, and the metal ring, and label in trunk for cars equipped with the "posi" controlled differential.  I will also add this to our Help Page.