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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
Mustang Trac-Lok Differential Spider Gears

What Do New Mustang Rear Gears Do?

Your rear differential gears dictate the final drive ratio which is denoted as a number such as 3.73, 4.10, 5.16, etc. This number is the amount of times the drive shaft will turn a full rotation before the wheels will turn one revolution. The steeper the ratio (the higher the number), the more power going to the wheels per revolution of the wheel. For example, if you had 4.60 gears, your motor would do 4 complete revolutions plus 60% of a 5th revolution to make your wheels turn one revolution.

Mustang Ring and Pinion Gear Set

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.

Gear Ratio Chart with Pros and Cons

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.

2000 V6 AmericanMuscle Mustang

Speedometer Calibration and Transmission Shift Points

Changing the gears will make your speedometer read incorrectly. This can be annoying and can get you in trouble with the law! Also if you have an automatic transmission this will cause your shift points to be incorrect and could cause some serious problems that could lead to transmission damage. There are several methods to correct these problems though.

2014 Mustang GT Doing a Burnout at the Drag Strip

Do I Need to Tune When Installing New Gears in a Mustang?

You have a couple of options as far as correcting your Mustang's speedometer and odometer reading. Depending on the year of your Mustang, the correction can be done by installing a speed calibrator, reprogramming with a chip or tuner, or installing the adjustment gear available through Ford racing.​

Custom Tunes

A custom tune requires the use of a chip or handheld tuning device such as the SCT Eliminator, X3 PowerFlash, or LiveWire tuner. Your dealer will create a custom tune that specifically calibrates your speedometer and if you have an automatic, it will calibrate your transmission's shift points. This method will work on all Mustangs with computer controlled electronic gauges (1999 Mustangs).

LiveWire TS Mustang Tuner

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.

Mustang Speedometer Gear

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

  • 1987-1993 Mustang GT and LX - 8.8
  • 1994 GT - 8.8 

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.

Ford Racing Mustang LSD

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.