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

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  • Item 389949
  • MPN# M-4001-88355

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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 Details

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

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    14 Questions / 5 Answers

    Ask a Question

    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.

      Rob D

    Helpful (1)

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


    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.

      Rob D

    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)

    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


    • The Ford Performance IRS Loaded Differential Housing 3.55 will come with the standard traction lok differential. This is a tried and true offering from Ford and has been used in numerous track cars. The Torsen differential has so far been shown to be a very stout differential, but is much newer so much less is known about its capabilities and longevity. With the Whipple and sticky tires, a Traction lok, or an aftermarket diff may be the best bet for long term durability.

      Rob D

    Helpful (0)

    Customer Reviews (3) Write a Review

    Overall Rating 5.0

    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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