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Swapping from 3.73 to 3.55

221
173
MD
So after much consideration, I decided to go from 3.73s in my 2017 PP to 3.55. I find that 3.55 with Torsen are a hard find because they were only available for auto cars staring in 2018 models. It just so happens that someone in my local mustang FB group is looking for 3.73s for his 19 PP auto so this is perfect and he has lower miles than me. At least now I can go to 18s without an issue with gearing.

My question is what is needed to make this work? To my understanding, auto and manual cars have different pinion flange as I think it differs by transmission and years. Is his diff aluminum or iron? I cant seem to get a concrete answer by browsing. Thanks for any help!
 
From what I can find:
2015-2017
Automatic and Manual equipped vehicles differ in pinion driveshaft flange.
Automatics have an aluminum case, where manuals have an iron case.

2018+
Automatic and Manual equipped vehicles differ in pinion driveshaft flange (and differ in bolt pattern from the earlier S550s).
All PP Mustangs regardless of transmission choice have an iron case.

Ford makes this technical fit note between the Gen 2 and Gen 3:
2015-2017 Mustang comes with a pinion flange using a 95mm bolt circle and 113mm driveshaft pilot
2018+ Mustang comes with pinion flange with 93mm bolt circle and 110mm driveshaft pilot

Automatic to the left, Manual to the right (Images are from 2018+ bolt pattern)
1644520884583.png
1644520898887.png

If you're swapping over the entire pumpkin, sounds like you'd need to keep the matching companion flange with the car. You'd need a replacement pinion seal and nut (since it gets staked into place). The note about accurately writing down the pinion nut's pre-load before removal in the instructions below is important, since these still use crush washers, so if you go beyond the initial pre-load setting you'll have an incorrect pre-load. Removing a crush washer that was compressed too far requires the disassembly of the entire differential.
Procedure is outlined here: https://performanceparts.ford.com/download/instructionsheets/FORDINSTSHTM-4851-M8A.PDF
 
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The above is correct. Upside is aluminum housing is 20lbs lighter. Downside is you need to swap pinion flanges.

I had my pinion flange swapped and the diff howls something fierce now. Talked to a mechanic buddy and while he said it’s technically possible to swap flanges and get preload right in practice it’s extremely difficult. So be forewarned.
 
221
173
MD
From what I can find:
2015-2017
Automatic and Manual equipped vehicles differ in pinion driveshaft flange.
Automatics have an aluminum case, where manuals have an iron case.

2018+
Automatic and Manual equipped vehicles differ in pinion driveshaft flange (and differ in bolt pattern from the earlier S550s).
All PP Mustangs regardless of transmission choice have an iron case.

Ford makes this technical fit note between the Gen 2 and Gen 3:
2015-2017 Mustang comes with a pinion flange using a 95mm bolt circle and 113mm driveshaft pilot
2018+ Mustang comes with pinion flange with 93mm bolt circle and 110mm driveshaft pilot

Automatic to the left, Manual to the right (Images are from 2018+ bolt pattern)
View attachment 72515
View attachment 72516

If you're swapping over the entire pumpkin, sounds like you'd need to keep the matching companion flange with the car. You'd need a replacement pinion seal and nut (since it gets staked into place). The note about accurately writing down the pinion nut's pre-load before removal in the instructions below is important, since these still use crush washers, so if you go beyond the initial pre-load setting you'll have an incorrect pre-load. Removing a crush washer that was compressed too far requires the disassembly of the entire differential.
Procedure is outlined here: https://performanceparts.ford.com/download/instructionsheets/FORDINSTSHTM-4851-M8A.PDF
Amazing. Thank you very much! I’m not the most knowledgeable when it comes to this but it looks like I don’t have to take the diff apart to swap the flanges so that’s good. Thank you again
The above is correct. Upside is aluminum housing is 20lbs lighter. Downside is you need to swap pinion flanges.

I had my pinion flange swapped and the diff howls something fierce now. Talked to a mechanic buddy and while he said it’s technically possible to swap flanges and get preload right in practice it’s extremely difficult. So be forewarned.
Thanks for the tip. I rather have the iron housing and it looks like that’s what it comes in. Right now I’m just trying to get laps in at minimal cost so when I get to weight savings it’ll definitely be on my list.
 
71
129
Exp. Level
5-10 Years
Australia
Have you noticed much difference in 3.55 gears? Here in Australia our cars only came with 3.55 in both manual or auto in PP1, im looking into swapping to 3.8 with torsen in it so just seeing if it worth the gear swap or just stick with 3.55
 
221
173
MD
I haven’t done so yet. Others may have better feedback than me. I think you would have to drive your track(s) first and see where you are shifting and if you run out of RPMs right be fore a turn. I think @Scootsmcgreggor posted a very helpful conversion tool in another thread that broke down everything for me
 
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The best gear ratio conversion tool I've used is linked below. You can see the differences it will make in terms of speed per gear and also what your new rev drop with each shift will be.
I did 3.55 to 3.73 when I had my ecoboost drivetrain and it was noticeable but not huge. It would be more pronounced with a gt350 though because the higher redline will exacerbate the speed difference per gear.

But certainly the final answer will depend on desired shift points per track that you frequent. Unfortunately a quick change rear end isn't an easy answer because that's the best one.

 

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