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Ford Performance IRS Ring Gear and Pinion Kit - 4.09 Gears (15-20 All)

Market Price $324.99 You Save 12% ($38.00)
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$286.99 (kit)

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  • Item 399248

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    Features & Details

    • Made for Street and Track Use
    • 4.09:1 Gear Ratio
    • 8.8" Ring Gear Diameter
    • Made from Premium Forged Material
    • Requires Professional Installation
    • Warranty Not Included
    • Fits All 2015-2020 Ford Mustangs
    Boosts Performance. With the Ford Performance IRS Ring Gear and Pinion Set, you can feel a noticeable improvement in your Mustang’s engine performance and fuel efficiency. Indeed, its gear ratio allows you to accelerate faster and perform even better whether you’re on the street or on the track.

    4.09:1 Gear Ratio. This ring gear and pinion set is truly the performance driver’s gear set. It comes with a 4.09:1 gear ratio which helps increase the performance of your car while maintaining its overall efficiency. It also helps your Mustang accelerate much more rapidly without the need to consume more fuel.

    Highly Durable Construction. This high-quality IRS Ring Gear and Pinion Set has been manufactured out of highly durable forged steel which ensures long-lasting and quiet performance. It’s also coated with a corrosion-resistant black oxide finish for extra durability.

    Recommended Upgrade. We recommend using a handheld tuner to recalibrate your Mustang’s speedometer after changing the rear axle gear ratio of your car. This is to avoid acceleration issues from miscalibration.

    Attention 2010 and Newer Mustang Owners. Ford changed the inner rear gear pinion bearing in all 2010 and newer Mustangs with an 8.8-inch rear gear pinion bearing. To properly install this set, you will need a new 8.8-inch rear gear pinion bearing (Ford part# 9L3Z-4630-A). Please see drop down option above for more information.

    For Professional Installation. Installation of this IRS Ring Gear and Pinion Set can be complicated. Which is why AmericanMuscle recommends getting the help of someone with mechanical experience to properly install this set on your Mustang. For best results, follow the included instructions.

    Warranty. This product is sold “as is” and is available without any warranty from the manufacturer.

    Application. This Ford Performance IRS Ring Gear and Pinion Set has been designed to fit all 2015-2020 Ford Mustangs.

    Fitment: 2015 2016 2017 2018 2019 2020 Details

    MPN# M-4209-88409A

    CA Residents: WARNING: Cancer and Reproductive Harm -

    What's in the Box
    • (1) Ring Gear
    • (1) Pinion Gear
    Installation Info
    Tech Specs
    Will it fit my Mustang?

      Customer Photos (5) Submit Your Photo

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        Questions & Answers (11)

        5 Top Rated Questions

        I have a 2015 GT non PP with 3.31 gears. Will this kit be a direct replacement? What all will I need to purchase for the install?

        • This will be a direct replacement and will have everything needed for the install.

        Helpful (1)

        I have the aluminum housing with 3.15 gears currently. Can I swap these gears into this differential or do I need a different differential carrier, or is the aluminum 3.15 case not compatible?

        • 4.09 gears are not compatible with a 3.15 ratio differential you will need to use differential M-4204-MT or M-4204-M to run the 4.09 ring and pinion gear set.

        Helpful (0)

        How well will this work with my 2019 mustang with a 10 speed automatic trans?

        • These will work just fine with the 10r80 transmission.

        Helpful (0)

        Is this a direct fit for a 2019 Mustang with the PP1 and a Torsen diff

        • This would be a direct with if you have the Torsen diff yes.

        Helpful (0)

        I have the 2017 gt premium with 3.15 gears. Will I need anything else to change to the 4.09

        Post Your Question
        6 More Questions
        I want to go with these gears in order to get get great acceleration however I do have to be on the freeway driving 65-75 mph daily. Will these gears have me screaming on the highway or is it bearable? Thanks 1 answer
        If I have a 2018 Gt with the 10 speed auto and 3:15 gear ratio, will this application work as a direct replacement? 1 answer
        I have a 2018 M6 PP with 3.73 gears and 29” tall tires. What will the effect be on highway driving with these gears? 1 answer
        I have a 2015 stock mustang GT and i was looking at getting this and also the Ford Performance T2R Torsen Differential - 34 Spline 8.8 in, would these be able to be installed and run with no problems on the stock parts? 1 answer
        Will it work with updated LSD diff (, or only with a torsen? 1 answer
        It is not required to change base ecoboost differential for this gearing? 1 answer

          Customer Reviews (6) Write a Review

          Overall Rating 5

          out of 5 stars

          Installation Time:Full Day

          • Performance Gain 4.666667 4.7
          • Fit 4.833333 4.8

          • 5/5

            Installation Time: Full Day

            August 31, 2019

            Exactly what I expected!!!

            Was looking for more ft. torque off the line. That’s EXACTLY what I got! Not worried about using more of my power off the line v. the top of the powerband. I needed and recommend the Ford Performance Ring and Pinion Installation Kit. Makes for a perfect install. Also, have someone that knows what they’re doing install it. A 2018 is NOT your grandad’s Mustang!! I had my the dealership put it in. Paid more but the labor on the install is guaranteed (my 5.0 is still under warranty and Im still paying on it!!!). Getting 18.8 mpg on the 1st 900 miles post install. Cost me $1500 for BOTH parts AND labor. Excellent move all around.


            Year: 2018 Submodel: GT

            Helpful (0)

          • 5/5

            Installation Time: Full Day

            August 13, 2019

            Ring and pinion gear

            Install sucked lol but the improvement it made was well worth it.


            Year: 2018 Submodel: GT

            Helpful (0)

          • 5/5

            Installation Time: Full Day

            July 21, 2019

            Should have been a standard option

            I installed these gear in a 2016 Mustang GT. The torque is through the roof. If you are running the stock 235 tires you will need to replace them. First gear will roast them. If the car is rolling in 2nd gear and you go WOT, once it gets to about 4K rpm you will start spinning the tires through redline, and if traction control is on it will start to flash like mad. I recommend the installation kit with it. You do need to re-calibrate the ECM or it will not run right (15-17). I bought the Ford Performance pack 2 and it was easy to do. The speedometer does adjust with out it, but the rpm do not match the speed as programmed so you have to re-calibrate! My mpg Hwy is 24.5 at about 80mph (~2500rpm) with hills. My average is over 21mpg. I do not have a clear city estimate. City I got it down to 16.1 according to the clock in about 40 miles of driving but keep in mind I was running it harder and my tachometer saw 2500-4500 rpm on average and wide open full rpm many a times. You can also start the car in 2nd gear for daily driving, although, I still like 1st. Getting the haft shafts out was a pain. I found it easier to drop the cradle (getting the cradle out is easy) and disassemble everything. This can be a one man job, but it helps to have a helping hand. Summery: -Amazing torque and acceleration -You need to re-calibrate (15-17) -Stock 235 tire will not work as they loose traction -MPG are great -You can start in 2nd gear -You can do it with a jack and jack stands -You can do it alone, but help is always nice


            Year: 2016 Submodel: GT

            Helpful (0)

          • 5/5

            Installation Time: Afternoon

            April 17, 2018

            Should be standard

            I bought a base GT becouse I wanted to add my own upgrades, better suspension better springs, better gear ratio. If you have the performance pack GT it will actually cost you more to install them becouse you have to change out more equipment. This was a simple swap from my stock 3.31 in my 2017, the car has 350miles on it. The car pulls like its supercharged through all gears in comparison to the 3.31’s it’s ridiculously improved. 2018 ford no longer installs 3.31’s in any level GT minimum is now 3.55’s. If you want to increase torque and 0-60 times I recommend 4.09 over anything below 3.73’s if you already have 3.73’s I wouldn’t bother as the change is negligible.


            Year: 2017 Submodel: GT

            Helpful (17)

          • 5/5

            Installation Time: Weekend

            March 04, 2018

            How it should have been

            This ring and pinion makes my ecoboost pull like a GT, 4.3 seconds to 60 (in the winter). I’ve lost 3 MPG which isn’t bad and I have an OEM exhaust so I don’t notice any sound difference from higher revs. The engine is definitely working harder (increase of about 1000rpm on route 80 going around 75-80 mph is about 3000 rpm) but I don’t care much about fuel economy. Install was a pain, I recommend looking up half shaft install for the mustang as that gets those steps out of the way, drop the exhaust after the braided flex point to make it SO much easier. 4 bolts hold in the dif along with the driveshaft which is 6 (I believe) 10MM bolts. It is a cup so you need to pry from the pry bolt. The half shafts may get stuck in the diff, tie a metal wire around the half shaft against the diff and the other end to a giant hammer and swing (look up the video, it’s how I did it) I STRONGLY recommend filling the differential BEFORE you put it in unless you have a fluid pump and have the car level. I also recommend taking the differential to a shop as install with the differential already out of the car is 300-400 vs 1000 for taking it out included, but put aside an entire day to take it out and another entire day to put it back, expect 20 hours of removal and install, not hard, just slow. (Also definitely have a friend with you, the diff is HEAVY) DO NOT push the brake if you take the calipers apart, it will cause them to rupture. Undo the driveshaft bolts first if you have an automatic car as you need to push the brake to put it in to neutral and park back and forth roughly 3 times (2 times if you take off the exhaust for more clearance) All and all worth taking apart the rear end of your own car, really useful to know how to do. Great gears and definitely worth the money. I would do it all again twice.


            Year: 2017 Submodel: EcoBoost

            Helpful (15)

          • 5/5

            Installation Time: Afternoon

            November 30, 2017


            Was a bit iffy on getting because it’s going on my daily but I’m honestly really glad I got this


            Year: 2016 Submodel: GT

            Helpful (17)

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