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A Guide to Mustang Rear Gears

Written By: Andrew Cilio

A guide to help you understand the uses and benefits behind your Mustang's rear differential gears. Changing these gears will have a positive effect on both acceleration and fuel efficiency, depending on the ratio you choose.

American Muscle

Gearing Overview

Out of all of the parts you can modify on your Mustang, modifications to one part in particular stands out for being a simple and cheap way to enhance your driving experience. Which part am I talking about? I am talking about your Mustang's rear differential gears! Your Mustang's rear consists of your axles, the gear housing, a power transfer device (IE a spool, LSD, posi-unit, etc.), and the ring and pinion gear set which is the part that we will be talking about today.

  • Replacing a Mustang's stock rear gears with a higher ratio will give you quicker acceleration, greatly improving your 60 foot times--depending on what ratio you choose
  • A ratio of 3.55 will get you down the drag strip quicker than most stock ratios, but 3.73s or 4.10s are an even greater improvement and so on
  • Going with too extreme a gear ratio can result in lost traction and reduced top speeds, depending on your overall setup
  • For a street car that might also see the track, 3.55s, 3.73s and 4.10s gears are recommended, with 4.10s being the gear of choice for automatic transmissions

Why Would You Want To Change Your Mustang's Gear Ratio?

There are many reasons, but the most common on a late model Mustang is to get your car into the power band faster. When Ford switched from the 5.0L pushrod motor to the 4.6L modular motor, a common complaint was that the new motor was sluggish off the line. This is because the 4.6L does not start making its torque until higher up in the RPMs unlike the 5.0L. Changing to a steeper gear will get that pig out of its blanket, and really wake it up off the line.

What Gear Ratio Should I Buy?

On a stock car or mildly modified late model Mustang with common bolt-ons I personally recommend 4.10 gears. 4.10s will give your car more pep, with negligible impact on your vehicle's drivability. Another popular choice is 3.73 gears, but the significant performance increase of the 4.10 gears far outweighs the minute gas tank penalty. For heavily modified naturally aspirated engines, forced induction, and big-shot nitrous cars, that is actually a trick question. There are many factors that would come into play, and it is best to talk to the shops that did the work on your engine, transmission, and chassis to find out what ratio to use.

What Concerns Are Involved When Changing a Mustang's Rear Gears?

The Fuel Economy Question: For most performance enthusiasts, fuel economy is a non-issue. However many of you may drive your Mustang as your daily driver which is a big factor in your decision to go to steeper gears. People that frequently travel far distances at highway speeds will notice reduced miles per tank with steep gears such as 4.60 gears. This is because steeper gears will put your car at a lower speed for any given RPM. At the same time a car with a very shallow gear set, such as 2.80 gears, will have to use more power to accelerate from a stop. This'll reduce your gas milage if you do a lot of stop and go driving. Most late model Mustangs came with 3.27 or 3.55 gears stock which provide very reasonable fuel economy at highway speeds, but they are not peppy enough accelerating from a stop. A happy medium for most daily drivers is a 3.73 gear ratio.

What Gear Ratio Does My Mustang Have Stock?

Over the years, Mustangs have been offered with a myriad of gear ratios, so figuring out which ones are in your car can be tough. Especially with late model Mustangs, it can be difficult to figure out what gears your Mustang has since Ford has offered everything from 3.31's to 3.73's from the factory 2011 and up.Often, you can find the gear ratio stamped in to the differential cover or written on to it from the factory, but these aren't always visibile and can be difficult to locate. Using a vin decoder can help and as a last resort you can go to a dealer and have them check to tell you. Below is a breakdown of Mustang gear ratios by year:

1979-1993 Mustang Stock Gear Ratios1994-1998 Mustang Stock Gear Ratios1999-2004 Mustang Stock Gear Ratios2005-2009 Mustang Stock Gear Ratios2010-2014 Mustang Stock Gear Ratios*2015-2016 Mustang Stock Gear Ratios
2.26 (78-82 4.2L V8 automatics)2.733.27 (V6 and GT)3.31 (All V6s and automatic GTs)2.733.15 (V6)
2.47 (79 5.0L V8 automatics)3.08 (manual trans)*3.55 (Mach 1 and Cobras)3.55 (GT manuals)3.153.31 (V8 and EcoBoost)
2.73 (87-93 V8 manuals)3.27 (automatic trans)*

3.313.55 (Performance Package)*
3.08 (82-85 V8s)*

3.553.73 (Torsen Perfromance Package)
3.27 (87-93 V8 automatics)*

3.73 (2013 Track Pack and Boss models)
3.45 (4cyl Turbos w/ 7.5" rear)*

SpeedCal Devices

A SpeedCal is an electronic device that allows you to adjust the signal that is being transmitted from your transmission to calibrate for gear changes. This works on Mustangs with both computer controlled electronic gauges and non-computer controlled electronic gauges (1994-2004 Mustangs).

Speedometer Driver Gear: Swapping out the speedometer driver gear inside the transmission is the only way to adjust the speedometer in Mustangs with mechanical gauges. This method also works with some electronic gauged mustangs, although this method is not as accurate as the previous two methods.

Bearings and Install Kits

If your Mustang is over or is close to 30,000 miles, your bearings are worn to your old gears. If you have less than 30,000 miles, it is generally accepted that you can install gears using just the install kit and your current bearings. Personally, I always recommend getting the full install kit with new bearings. It is cheap insurance to prevent having to replace your bearings and other parts that the worn bearings damaged down the road.

What Size Rear Differential Does my Mustang GT Have?

8.8 vs. 7.5 Mustang Rear Ends

  • 1987-2018 Mustang GT and LX - 8.8
  • 1979-1985 V8 - 7.5
  • 1986-2018 V6 & EcoBoost 7.5

Should I Rebuild my Mustang's Rear Differential?

The whole point of a differential is to get power to tires with traction. Limited slip differentials make it so your Mustang can use both rear tires to push forward rather than one, making winter months a bit easier to deal with. LSDs are also popular for drifters since both tires spin in a drift which not only looks cooler, but makes it easier to swing out the back end. The easiest way to tell if your Mustang's LSD needs a rebuild is to do a burnout and have a friend tell you if both tires are smoking or if just one is spinning. If it's just the one, it's time for a rebuild.

What Size Rear Does my Mustang Have?

From 1979-1985 Mustang's came with the 7.5" rear. Up until 2011 the 7.5" was factory for everything without a V8. The 8.8" rear was the differential of choice for the V8 engines (4.6, 5.0, and the 5.4). After 2011 though the 3.7L V6 and the Coyote powered Mustangs shared the 8.8". The Mustangs with IRS (Independent Rear Suspension) under them also came with the 8.8" rear end (99-01 and 03-04).

Granted, people change out the rear differentials for a number of reasons. One way to tell the difference is by the cover gasket and the pinion gear splines. The 7.5" rear has two flat edges on the gasket whereas the 8.8" is mostly round with two divets. Lastly the 7.5" pinion has 28 splines, and the 8.8" has 30 splines.

V6 Mustang Differentials and Interchangeability

Up until 2011, the V6 models had different rear ends than their 8 cylinder counterparts. A 2010 older V6 Mustang has a 7.5 rear end. As a result, they have their own gears (although the same size at the 8 cylinder 8.8 rear ends). Therefore, you can use V6 gears on all V6 Mustangs from 1994-2010. However, a 2011-2014 gear from a V6 will not be compatible with a 2010 and older model. V8 models use 8.8 rear ends and are interchangeable between All V8 models and V6 models from 2011 to the current production year.

Do I need a Shop to Install Mustang Gears?

Changing rear gears on any vehicle is a very complex task. While it is straight forward upon first glance, the precision and time it takes to complete the job can be overwhelming. If one does not utilize the proper equipment (measuring device, torque wrench, beam type torque wrench, slide hammer, etc) the job can be almost impossible to do correctly. If installed incorrectly, gears can have improper, premature wear. As a result from improper installation, they can also whine and be quite noisy. Improper installation is easier to accomplish than a correct install of the gears; a simple micrometer off the correct backlash can cause improper wear.

Mustang Gearing For Autocross and Road Racing

Choosing gears for autocross/road racing can be similar, yet different from one another. One major item to consider is the course itself (one will primarily be racing on). The general idea is to remain in gear for the majority of the track; shifting as little as possible. Shifting consistently slows down times by not putting the power to the wheels. Somethings to consider for proper gear selection can be items as average course speed, distance, the degree of turns, etc.

3.73's tend to be a popular choice, but it really comes down to a variety of factors. Your best bet is talking to people with similar Mustang's to yours to get an idea what works best.

How Much Gear Oil Does My Mustang's Diff Hold?

On Mustangs from 1986-2014 (with the exception of the 1999-2004 Cobras) utilize the 8.8 rear end. This fluid capacity is roughly 3 quarts (including a bottle of friction modifier). For 2015 and newer models, the fluid capacity is 1.6 quarts and 4 oz of fluid capacity. This does not apply to the GT350 as it holds slightly more fluid. On V6 models until 2011 (in 2011, they received an 8.8 rear end) the capacity is the same as the 8.8 counterpart.

Fitment includes: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, GT, V6, Cobra, ShelbyGT500, Mach1, Bullitt, Boss, LX, SVO, EcoBoost