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Ford Performance IRS Loaded Differential Housing 3.55 (15-18 GT, V6)

AmericanMuscle no longer carries the Ford Performance IRS Loaded Differential Housing 3.55 (15-18 GT, V6). Please check out 2015-2019 Mustang Limited Slip Differentials for an updated selection.
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Features & Details

  • Original Equipment on Manual GT Mustangs
  • Iron Housing with Aluminum Cover
  • 8.8" LOK Limited Slip Differential
  • Pre-Loaded with 3.55 Gears
  • Requires 34 Spline Axles
  • Genuine Ford Racing Part - M-4001-88355
  • Direct Fit Upgrade - Tune Required for Speedometer and Shift Corrections
  • Automatics will require Pinion Flange Kit part# M-4851-M8A
  • Fits 2015-2018 GT and V6 Mustangs
Original Equipment on Manual GT. When the light turns green, allow your V6 or GT S550 to accelerate off the line faster by installing a Ford Racing 3.55 Differential. This Ford Racing 3.55 IRS Differential Housing improves the low end torque of your Mustang for stronger, quicker launches. So whether you want to upgrade your gears or replace a broken housing, you can't go wrong with a direct fit replacement differential that was standard on manual GT Mustangs from the factory. This fact makes the manual iron differential the perfect upgrade replacement for automatic Mustangs that see a lot of track time.

Pre-Loaded. Ford Racing pre-installs a set of their 3.55 ratio gears and a LOK Limited Slip Differential into an iron IRS housing and tops it all off with an aluminum cover. This LSD is ready to be installed straight out of the box.

Traction-LOK Limited Slip Differential. The included heavy duty Ford Racing Traction-LOK Limited Slip Differential (LSD), sometimes referred to as a "Posi", allows both rear wheels to put the power down to the pavement as needed. If one of the drive wheels begins to slip or lose traction, the LSD transfers more power to the nonslipping wheel to provide better traction to keep you moving. This makes Limited-slip Differentials ideal for street, autocrossing, road racing and impressive burnouts.

Ford Racing 3.55.1 Gears. The pre-loaded Ford Racing ring and pinion gears have a 3.55:1 gear ratio and 8.8" ring gear diameter. These gears are manufactured in the USA and feature a super strong forged construction with CNC cut & lapped gear teeth. They are completed in a black oxide finish for corrosion resistance.

Required Upgrade. The Ford Racing 3.55 IRS Differential will require reprogramming the vehicles computer for speedometer and shift point corrections. AmericanMuscle recommends a Bama custom tune with a Mustang Handheld Tuner to reprogram the vehicles computer (ECU) for proper operation.

Automatic Transmission Requirement. Installation of this Loaded IRS Differential Housing on a S550 Mustang with an automatic transmission will require the Automatic IRS Pinion Flange Kit part# M-4851-M8A for proper installation. 

Application. This Ford Racing 3.55 Loaded IRS Differential Housing is a direct fit upgrade replacement for the 2015 to 2018 GT and V6 Mustangs. Designed for use with independent rear suspensions (IRS) and requires the use of 34 spline axles.

Fitment: 2015 2016 2017 2018 2019 Details

CA Residents: Warning: California's Proposition 65

What's in the Box
  • (1) Loaded IRS Differential Housing
Installation Info
Tech Specs
Will it fit my Mustang?

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Questions & Answers (14)

5 Top Rated Questions

If I install diff in 2016 auto , can I change the driveshaft to Driveshaft Shop manual and not have to change the pinion flange ?

  • The Ford Performance IRS Loaded Differential Housing 3.55 will come with the manual trans pinion flange, so no pinion flange swap would be needed.

Helpful (1)

Hello, Excuse what do you mean requires the use of 34 spline axles?

  • This differential is only going to be compatible with 34-spline halfshalfts. The factory halfshafts are 34-spline, so the factory ones would work with this differential.

Helpful (0)

Do they offer a loaded housing with 3:73 ratio Thanks

  • We currently would not carry the Ford Performance IRS Loaded Differential Housing with the 3.73 gears no.

Helpful (0)

Is there any structural improvement within this housing over the stock GT housing? Looking to upgrade to 3.55 or 3.73 gears. However, if this will add further structure, I'd rather choose the complete package.

  • Ford Performance IRS Loaded Differential Housing will use the OEM GT housing. There will be no structural improvements over the stock housing.

Helpful (0)

I purchased a GT350 Voodoo intake and CAI but am not getting up into the new intakes sweet spot fast enough with my 3.15 auto transmission. Would this help with acceleration enough or can I get it loaded with different gears? Also thinking about adding headers but can’t decide which gives me more bang for my buck.

  • Hello, ChuckJT5U3961325 If you're looking for faster more aggressive acceleration I would recommend going with a different gear ratio, the one included in the Ford Performance IRS Loaded Differential Housing 3.55 is a 3.55. With that being said I would recommend a 3.73 gear ratio, so you would need to swap out your stock diff with the Ford Performance T2R Torsen Differential (390234) and couple that with a set of Ford Performance IRS Ring Gear and Pinion Kit - 3.73 Gears (384403).

Helpful (0)
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9 More Questions
I have the torsen 3:73 pp car I want to drag race most of the time, what do you think I should do I have been dragging the car often best 60 foot is 1.4,ran a 10:55 @128 Whipple stage 2 with stock exhaust and whipple tune I'm afraid the torsen will break starting to make noise 1 answer
I have a performance pack 2015 manual with a Whipple stage 2 is there any gains to putting the lsd3.55 in and what about weight difference 1 answer
What is the torque load and horsepower rating that this differential can Withstand, also are there products available to increase those ratings for 1400 hp drag car engine and transmission? 1 answer
I have a 2016 v6 mustang with an automatic transmission, is anything else required for this to fit ?? 1 answer
This is a follow up question regarding The Roush tune. I understand the retune is necessary when changing the rear gears for speedometer correction. My question pertains to the shift changes,throttle mapping,etc that the Roush tune does for the car. Will these changes carryover to the new gears in relation to the speedometer/tachometer activity? 1 answer
I have a dealer installed Roush level one tune on my 2015 V6 which has 3.15 gears on an auto transmission.If I put this 3.55 on my car will the necessary retune keep the shift points the same that the Roush tune intended? 1 answer
I have an ecoboost mustang which I see isn't listed for this diff. What would stop it from working? 1 answer
Can I get 3.73 gears put in instead of 3.55's 1 answer
I just bought a 2017 V6 convertible Mustang with a manual six speed and 3.15 differential. I want to change the diff to the 3.55 like the one in my 2016 Fastback GT. Will this conversion fit or is there modification or different axles needed? 1 answer

Customer Reviews (3) Write a Review

Overall Rating 5

out of 5 stars

Installation Time:Shop Install

Installation Cost:$250-$500

  • Performance Gain 5 5
  • Fit 5 5

  • 5/5

    Installation Time: Shop Install

    Installation Cost: $250-$500

    June 29, 2017

    awesome product

    Got this to replace the stock 3.15 gears and aluminum housing and it works beautifully... car takes off quickly


    Year: 2015 Submodel: GT

    Helpful (11)

  • 5/5

    Installation Time: Shop Install

    March 18, 2017


    When I purchased the Ford Performance Differential with 3.55 gearing to replace the 3.31 that came with my 2016 GT/CS, I was a little nervous...but after the exchange, I noticed quite a difference right away! The car was fast before but not as quick as now. I had a Ford dealership to do it. The guys there are Mustang enthusiast! Cost was around $700. It's been a little more than a week now and I am very satisfied.


    Year: 2016 Submodel: GT

    Helpful (12)

  • 5/5

    October 02, 2016


    This did the job perfectly just as advertised. Buddy and I installed this ourselves couldn't be happier.


    Year: 2015 Submodel: V6

    Helpful (9)

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