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

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

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:

 

 

Views: 14193

Comment

You need to be a member of 63/64 Cadillac Website to add comments!

Join 63/64 Cadillac Website

Comment by Jason Edge on October 17, 2017 at 7:51am

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

Comment by Jason Edge on September 19, 2017 at 8:47am

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 #!

Comment by Jason Edge on September 6, 2017 at 8:36am

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.

Comment by Jason Edge on September 5, 2017 at 9:36pm

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.

Comment by David Thomas on September 5, 2017 at 8:46pm

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.

Comment by Jason Edge on November 23, 2012 at 10:12am

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.

Photos

  • Add Photos
  • View All

Forum

Fender Trim Removal

Started by Kevin Wray in Restoration Discussion. Last reply by Jason Edge yesterday. 6 Replies

Thanks to Jason's expert advice, I was able to remove the fender trim on my 63 de Ville without removing the fender. Pics contain the locations of the 3/8 in nuts and how to access them. My clips…Continue

Tags: removal, trim

Body side molding install help needed

Started by Wayne Morris in 1963/64 Cadillac Specific Discussion. Last reply by Jason Edge yesterday. 3 Replies

I am in the process of trying to install body side molding. I have completed installing the two front fender pieces and now trying ti figure out the rest.…Continue

ORIGINAL CAR

Started by David Erickson in General Discussion yesterday. 0 Replies

I have an all original 63 Cadillac Park Avenue.  It has won awards in the past in the Originality Category.  My question to the group is what types of work you can do on the car that does not cause…Continue

Current 1963/64 Cadillac Listings on eBay

Started by Jason Edge in 1963/64 Cadillac Specific Discussion. Last reply by Jason Edge on Wednesday. 1 Reply

To see all 1963/64 Cadillacs Listed on eBay in the USA…Continue

Tags: ebay

Blog Posts

New old stock seat material

Posted by Ray Schick on August 22, 2024 at 6:25pm 2 Comments

I bought this seat material thinking that it’s…

Continue

Loud air moving sound within carb with cleaner removed

Posted by Dave Fehrle on March 23, 2024 at 3:23pm 8 Comments

While checking my Carter carb with engine running, I took the air cleaner off and there was a very loud hissing sound of air. I placed a piece of paper over the carb and it didn’t move up or down, is this normal?

Vintage Air AC

Posted by Chase on March 21, 2024 at 5:55am 0 Comments

Hey everyone, has anybody done a Vintage Air AC/Heat setup in their car? I want to bypass the old vacuum setup and go with a Vintage Air Gen 2 model. Before I get started, does anyone have any experience with this? (Which model to go with, do’s and dont’s, etc.) It looks pretty involved. Any and all info/pics would be greatly appreciated. Thanks!

accumulator dryer

Posted by CHRIS CARLSON on December 6, 2023 at 10:54am 0 Comments

Who rebuilds original AC  accumulator dryers for 1964 Cadillac? I have been on Classic auto air in Tampa FL wait list for over a year ,  There is one on Old air products web site that looks very similar to the original # 21-4214A but states it fits 1962 Gm cars, The one recommended for a 1964 Cadillac is half the length 21-4215A    What  is everyone using ? I would prefer to be as original as possible. Thanks

Videos

  • Add Videos
  • View All

© 2024   Created by Jason Edge.   Powered by

Badges  |  Report an Issue  |  Terms of Service