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Mustang S550 Differential Options

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One of the most rewarding upgrades a Mustang owner can install is upgraded gears. With a new gear ratio, you can dial in the exact driving experience you are looking for on and off the track. Whether you are an avid enthusiast or an experienced track star any driver can greatly benefit from the personalized diving experience when putting the pedal to the metal.

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With the introduction of the S550 platform for model year ‘15, Ford made the independent rear suspension (IRS) standard equipment across all Mustang models. This was an unprecedented decision for the original pony car (with the exception of the 99-04 Cobra) and brought the refinement of European sports car handling to the iconic American Mustang platform. With this new suspension design brings a new differential, which Ford calls the Super 8.8. The ring gear size is the same as its predecessor, hence the 8.8, but that is where the similarities end. Let’s take a look at the S550 differential options.

What is a Differential?

Before we get into the options for the S550, let's give a brief description of what the differential is. Turning force is transmitted from engine to the transmission and from there it flows through the driveshaft at different speeds depending on what gear is selected. 

From here it needs to get to the tires and to the road. In order to do this, the power flows through a differential where power is split between the rear wheels. The amount of power transmitted and the speed at which the wheels turn is dependent on the type of differential and the ratio of the gear set.


34 Spline 8.8" Limited Slip IRS Differntial

Final Drive Ratios - Lower or Higher?

The ratio of the differential gear set has a big impact on how a car behaves and can give the driver a different impression of its capabilities depending on the selection.  A lower ratio (higher numerical) will give the driver a feeling of more direct power and snappy acceleration. 

This normally comes at the expense of fuel mileage and engine wear if high speeds are maintained for a while. This is because the engine will be spinning at higher RPMS to maintain interstate speeds. 

A higher gear ratio (lower numerical) will not give the feeling of brutal acceleration but it will allow for a higher top speed and a lower RPM when maintaining these speeds. Because of this, lower ratio gear sets aren’t standard. However, as the Mustang is a car aimed the performance minded driver, lower ratios are available as options. 


4.09 Ring & Pinion Kit

Factory Options

Ford introduced the S550 platform in the USA with three powertrain options: the 3.7-liter V6; 2.3 liter-I4 Ecoboost; and the powerful 5-liter V-8. Every model comes equipped standard with a Trac-Lok limited slip differential. Each powertrain option has different gear ratio options available depending on your preference for auto or manual. 

 

3.7-liter Ti-VCT V6

2.3-liter EcoBoost I4

5.0-liter Ti-VCT V8

3.15:1

3.15:1 (auto only)

3.15:1 (auto only)

3.55:1 (option)

3.31:1 (standard on manual, option on auto)

3.31:1 (standard on manual only)

 

3.55:1 (Performance Pack)

3.55:1 (option)

   

3.73:1 Torsen (Performance Pack manual only)

Difference Between Trac-Lok (clutch-type) and Torsen T2 (parallel gear) 

All S550 Mustangs arrive with a limited-slip differential. This allows for better power transmission to the pavement.  A standard open differential has no way to limit power from a slipping wheel and transfer it to the wheel with traction. 

The Trac-Lok differential uses clutches to vary, or limit, the power from a slipping wheel and transfer it to the wheel with more traction. This lets the Mustang deliver all of its power more effectively and give the car greater acceleration. The Trac-Lok also allows for the speed of the wheels to vary as the car is turning and neither wheel is slipping.

An option on the 5-liter V-8 is the Torsen T-2R which comes with the selection of the Performance Pack option. Where the Trac-Lok uses clutch packs which tighten up upon higher torque loads closely coupling the drive wheels together, the Torsen uses internal gears to achieve the necessary friction. 

Through a complex set of internal gears, the transmission of force from the wheel with less traction is practically instant and fully locks the drive wheels together. Not only is the Torsen more performance oriented, the lower 3.73:1 ratio increases the Mustang’s acceleration making it a real muscle machine right from the factory. 

For these reasons, anyone looking to get the most out of their Mustang for high performance driving, auto-cross, and road courses will want to go with this option.

Available Aftermarket Options

For those who are not satisfied with their current gear set or are wanting something more extreme than available from the factory, the aftermarket has your solution. Ford Performance offers ratios of 3.31:1, 3.55:1, and 3.73:1 for the Super 8.8.

In addition, there is a 4.09:1 ratio which according to the manufacturer is intended for off-road use only. This would be an extreme choice for the street but would seriously increase the acceleration. 

Also available from the aftermarket is the same Torsen T-2R which comes with the Performance Package so you can upgrade to this more track-capable offering. Note a change in ratio will require a reprogrammer for speedometer calibration and shift corrections.

Cast Aluminum vs Cast Iron Housing

The S550 Mustang is a high-performance car and was designed from the factory to withstand these conditions. However, once you start adding power, a weak point makes itself known and that is the differential housing of the automatic cars. 

Ford equipped the automatic Mustangs with a cast aluminum housing which has mounting points subject to failure under severe usage. For those who have broken one or want to upgrade, a cast iron housing is available with 3.55:1 and 3.73:1 with either a Trac-Lok or the Torsen.


Iron Housing 3.55 IRS Differential Housing
Fitment includes: 2015, 2016, 2017, 2018, 2019, GT, V6, EcoBoost, ShelbyGT350