Out of all of the parts you can modify on your Mustang, modifications to one part overall stands out as being a simple and cheap way to enhance the driving experience of your car. 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.
What Do 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 is 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.
Why Would You Want To Change Your Ratio? There are many reasons, but the most common on a late model Mustang is to get your car into your 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 much higher RPMS then 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 Ratio Should I Buy? On a stock car or mildly modified late model Mustang with common bolt-ons I personally recommend 4.10 gears. 4.10 gears will give your car much 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.
For most performance enthusiasts, fuel economy is a non-issue. But many of you may drive your Mustang as your daily driver making it 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 very steep gears such as 4.60 gears. This is because steeper gears will put your car at a lower speed for any given RPM. But 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, making them see a reduction in miles per tank if they 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 just are not peppy enough accelerating from a stop. A happy medium for most daily drivers is a 3.73 gear ratio.
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.
Custom Tune: 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).
SpeedCal Device: 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 car 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 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 the parts the worn bearings damaged down the road.
What Size Rear Do I Have? If you have a V8 equipped Mustang, then your car should have come with 8.8" Rear Gears from the factory. V6 models until 2010 came with 7.5" Rear Gears from the factory, in 2011 V6's received the same 8.8" as the V8 models.
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