Rear end ratios

[Desotodav]

and some others...

 

[ggdad1951]

my rear diff had NO stamps...I understand your frustration, but most look to be 4.1.  Best to just to a rotation count.

[TrampSteer]

That last extra numbers look like a date. They all look like 4.1 IMO except for that one with just a triangle left on it (maybe a 4 also).

[Phil363]

I am still in the "very early" stages of breaking down my 52 Dodge B-3-B to the frame.  This thread and several others have me interested in going to a 3.73 diff from a '50 Plymouth in my truck as well. 

 

Just thinking it would be easier to acomplish with everything apart.

 

I attached my differential photo showing 4.1F .. no clue what the F means but the 4.1 mirrors what is on my chrysler build car for the axle ratio that was on the truck when sold originally.

[John-T-53]

_{Back in 2009 I brought up the same question and here's a reply I received from "DodgeB4YA" (Bob)...}

 

_{John,
You need to look for 1953 to 4 Plymouth rear end pumpkin-they use a 16 spline axle shaft which is what your 1953 B-4 1/2 ton has. Up to 52 they use 10 spline. You will have to swap the driveline yoke also. I say Plymouth because they are usually easiest to find. Dodge 1953 to 4 also will work as long as it's a 6 cyl. car-NO wagons or long wheel base cars. They use a bigger rear end (Pumpkin). On the bottom of the rear carrier is the ratio stamped on a 2" flat pad-need to clean it well to read for the ratio. The truck is the same.
Good luck, Bob}

 

_{I searched for a couple years and couldn't find any pumpkins out there. Maybe for 1952 and earlier years they are more plentiful. So I scrapped the idea and changed the tranny instead...}

Edited September 11, 2014 by John-T-53

[Desotodav]

Now I am just plain confused...

I chatted with a friend of mine who happened to have quite a few old MOPAR car diffs (8 & 1/4 inch Chrysler Royal) laying around, so he told me to come and take my pick. All going well so far... until I paid him a visit and inspected his diffs. I found a couple of them stamped 3.9 over 10... and then I spotted a few stamped 3.9 over 11. Could it be that the 3.9 over 11 (pictured) is actually a 3.54 ratio after one does the maths?... and that old mate in the diff factory got lazy thinking that rather than having to stamp a '.54' after the '3' that he'd just stamp an '11' rather than a '10' for the fraction number?... I'm sure hoping so as that might mean that I have a 3.54 diff rather than a 3.9 !!! I added some black ink to the numbers for the photos to make them easier to see.

Check out the fancy chrome one that I brought home also (pictured) which was just stamped '3.9'

Edited September 20, 2014 by Desotodav

[Young Ed]

The only way to know is pull the diffs and find the stamp on the ring gear or if thats missing count the teeth.

[Jeff Balazs]

Can't you just spin them and count the revolutions? It should be pretty easy to see the difference between a 3.5 and a 3.9. Might need to make a few reference marks but it should be doable.

Jeff

[Dodgeb4ya]

Look like 3.9's to me.

[Desotodav]

Thanks for the replies. The brakes are binding from sitting for too long so neither of the diffs stamped 3.9 over 11 will spin. I'll clean them up if I get time after the car show today and maybe pull the hubs. I suppose that 3.9 will be a little better than 4.1 if that is what they turn out to be.

[DJ194950]

Looking at my Hollander reprint for these years shows that any 49-52 Mopar 6 cyl.. motored vehicl should work. But, the exceptions are the 8 pass. cars and Suburbans(woody) only.

 

Now to find a 3.73 ratio.  Some 3.54 also out there - look for Chrysler 3rd. members. Common (standard) 3.73 ratio in 49-50 bus. cpes..

 

Your axles must be of the 10 spline type to work with these, but yours is supposedly a 10 spline stock.

 

If you pull one and it's a 16 spline axle, let me know. The interchange show other Mopar that use that. Mostly later models.

 

DJ

[DJ194950]

Desotodav,

 

If that 3-9 over 11 is on the ring gear the Hollander manuel says that it a 3.54 ratio, a 41-11 is a 3.73, 41-10 is 4.1, 43-10 is 4.3, 43-9 a 4.78.

 

Again the #,s quoted are on the Ring gear per book. Also used on the case?? Australian, and/ or export only??

 

DJ

[Desotodav]

Thanks for the replies gents.

I received confirmation today that the two diffs that I have which are stamped 3.9 over 11 are definitely 3.54 ratio. I must say that I consider it strange to see them stamped in that way rather than just plain old '3.54'. Needless to say that I am smiling like a kid with a new toy now knowing that I have 2 diffs with that ratio. I'll get them down to the diff shop soon for refurbishing.

[Dodgeb4ya]

Well I've junked at least 10-12  3.9's

Oh well live and learn.

[HanksB3B]

Thanks for the replies gents.

I received confirmation today that the two diffs that I have which are stamped 3.9 over 11 are definitely 3.54 ratio. I must say that I consider it strange to see them stamped in that way rather than just plain old '3.54'. Needless to say that I am smiling like a kid with a new toy now knowing that I have 2 diffs with that ratio. I'll get them down to the diff shop soon for refurbishing.

 

oooh,

 

Are they ten spline or 16 ?

 

Hank 

[Dodger]

Has anyone determined what a 41F is?  I'd like to know what the ratio on my 53' truck might be.

[John-T-53]

4 minutes ago, Dodger said:

Has anyone determined what a 41F is?  I'd like to know what the ratio on my 53' truck might be.

 

I haven't heard of that term, but you probably have a 4.10:1 ratio. You can look in the factory shop manual for your exact year and model and it will list the rear end that was standard for your truck. 

[Dodger]

I would but ... i don't have a shop manual yet.  It's on my list.  I have a 53 B-3-C.

[John-T-53]

There might be, somewhere on this site, a factory shop manual. I just did a quick search but couldn't find it in the scanned document section (see link below). 

 

I don't have mine handy at the moment, but perhaps someone might know if we have a B3 manual on here? 

 

https://p15-d24.com/files/

[JBNeal]

2 hours ago, Dodger said:

I would but ... i don't have a shop manual yet.  It's on my list.  I have a 53 B-3-C.

FSM lists 4.1 or 4.78 as possible rear axle ratios available...the extra letter may be a mfg code, such as a revision mark for the axle casting.

[Dodger]

I've never worked on rear axles.  Can I just change a gear in the rear to make the axle a 3.7 or 3.5?  If so, where would I find the gears?

[John-T-53]

No, you can't. There are no gears made for these rear ends. You have to change out the entire rear end. There have been some projects of the like that have been discussed on here. I believe a late model Jeep Cherokee was one such candidate for a replacement rear, but I'm not an expert in aftermarket rear ends. 

A mopar 8-3/4" would also be a good choice if you found one from a carline with the right width (A-body or B-body).

[Los_Control]

On 2/24/2025 at 5:04 PM, Dodger said:

I've never worked on rear axles.  Can I just change a gear in the rear to make the axle a 3.7 or 3.5?  If so, where would I find the gears?

Reading your first post, you are correct about changing the center section only.

You are also correct about spline count on the axles .... Like you, I was told to count them to see if they are the same.

Best advice I would have is to pull one of your axles and count before you go to the wrecking yard shopping.

I have never seen that information listed here as to what vehicles used which axles ... just count is all I've seen.

 

I did this swap on a 1961 Dodge truck before, it was really easy for me and only took a couple hours to complete.

I was installing the original 4.11 posi that the previous owner took out and put in too tall highway gears ... so everything just fit for me.

 

I would prefer a modern swap though myself, 3:73 gears, modern u-joints, self adjusting brakes, non tapered axles .... there is a lot of winning going with a modern rear end.