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jerrybev

Posted 2023-07-24 10:42 AM (#630917)
Subject: 1956 Plymouth rear end number?

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Location: Winston Salem, N.C.

I am seeking to improve gas mileage. I am a senior citizen and drive as gently as possible and get 11 mpg. Wondering if the housand or something can be changed to improve gas mileage. It is a 56 blevedere 4 door station wagon, automatic transmission. with a 318A 1958 engine in it. serial # 26556317

Does anyone have access to information to reveal what rear end gear it has? The tag on the rear end is missing. Would it be a 8 3/4 rear end?

Whatever rear it has, would a later model one fit in ?

thank you for your time and help
jerry whitfield in N. carolina

(1956 ply rear xxxsmall.jpg)

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1956 ply rear xxxsmall.jpg (111KB - 51 downloads)

56D500boy

Posted 2023-07-24 2:53 PM (#630929 - in reply to #630917)
Subject: RE: 1956 Plymouth rear end number?

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.
Jerry: I think in those years 55-56 the estate wagons of the senior cars (Chrysler and Desoto) would have had 8 3/4" rear ends. However, I think your 56 wagon would only have the 8 1/4" rear end.

The axle ration is probably stamped on the nose of the pinion carrier as shown in the photo below. I would guess that the ratio would be 3.54 but it might be 3.73. You can confirm this by checking the stamping and/or jacking one wheel up safely (with a jack stand for safety) and, with the eBrake off and the transmission in Neutral, count the number of rotations of the drive shaft per one rotation of the elevated rear wheel.

58coupe

Posted 2023-07-25 12:14 PM (#630944 - in reply to #630917)
Subject: Re: 1956 Plymouth rear end number?

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The 8 1/4" rear diff. was used in the 6 cylinder cars until I believe 61 but pretty sure you won't find a ratio lower than 3.31 which would be some improvement. The 8 3/4" rear used in the V-8 Plymouths from 57 on and also Dodge, Desoto, and Chrysler has a lot more options but is not an easy bolt in.
You have to change the complete housing and the width is different. Search in the archives to see what others have done.

Powerflite

Posted 2023-07-26 2:23 AM (#630966 - in reply to #630917)
Subject: Re: 1956 Plymouth rear end number?

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If you want to put a later 8 3/4 into it, you will need a '65-'67 B-body rear. The '68-'70 B-body will work too, but with more custom offset on your wheels. You will need to remove your original spring mounts, clean them up, and weld them to your new housing at a 41" separation. You will also need to make a new driveshaft to fit up to it.

jerrybev

Posted 2023-07-26 2:52 PM (#630978 - in reply to #630929)
Subject: RE: 1956 Plymouth rear end number?

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Posts: 689
500

Location: Winston Salem, N.C.

hi
elevated vehicle and verified my rear end # 1141544N1. Can you verify that this is a 8 3/4 or 8 1/4 based on the #? Mine is an 11 bolts in housing, either 339 or 389 is on side.
thank you
Jerry

Powerflite

Posted 2023-07-26 4:24 PM (#630980 - in reply to #630917)
Subject: Re: 1956 Plymouth rear end number?

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I have no idea what that number means, but a '57-up 8 3/4 has 10 bolts to mount the 3rd member, so if yours is 11 bolts, it isn't an 8 3/4.

Edited by Powerflite 2023-07-26 4:26 PM

jerrybev

Posted 2023-07-28 10:38 AM (#631014 - in reply to #630966)
Subject: Re: 1956 Plymouth rear end number?

Elite Veteran

Posts: 689
500

Location: Winston Salem, N.C.

hi
thank you for your time. Others are telling me my rear end is 8 1/4 since it has 11 bolts in housing. Therefore, what would I need.
thank you
jerry

Viper Guy

Posted 2023-07-28 11:23 AM (#631017 - in reply to #630917)
Subject: Re: 1956 Plymouth rear end number?

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Location: Branson, MO

At the minimum you would need a ring and pinion with a ratio in the 2.9:1 or 3..2:1. I think a 2.7:1 might be a little too light for your car.

You also would need the gaskets and seals.

Good luck.

57chizler

Posted 2023-07-28 6:06 PM (#631021 - in reply to #631017)
Subject: Re: 1956 Plymouth rear end number?

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Location: NorCal

Look at Dave's snip from the parts book, looks like 3.54 was the lowest numeric ratio available.

56D500boy

Posted 2023-07-28 8:11 PM (#631022 - in reply to #631021)
Subject: Re: 1956 Plymouth rear end number?

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57chizler - 2023-07-28 3:06 PM
Look at Dave's snip from the parts book, looks like 3.54 was the lowest numeric ratio available.

At least for a 55-56 Dodge/Plymouth.

(55-58RearDiffGears_2.jpg)

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55-58RearDiffGears_2.jpg (149KB - 55 downloads)

57chizler

Posted 2023-07-29 12:42 PM (#631032 - in reply to #631022)
Subject: Re: 1956 Plymouth rear end number?

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Location: NorCal

The early 8 1/4" was used from '37-'61 and in all those years the available ratios ranged from 3.54 to 4.78.

The '62-'65 version offered 2.93 and 3.31 ratios but they won't interchange with the earlier version.

Then, to confuse matters even more, they introduced a third version of the 8 1/4" in '69.

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