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Rear Axles
The ’94-2004 Mustang came with three main rear axle variants. All the V-6 cars had a 7.5-inch ring-
gear solid axle, one width of housing from ’94-’98, and then a wider track from ’99-2004. The ’94-’
98 Mustang GT and Cobra had the 8.8-inch ring gear solid axle, while the ’99-2004 Mustang GT
and Mach 1 had a wider track version of the same solid axle. Of course, the ’99-2004 Cobras had
an independent rear suspension that was still based on the 8.8-inch ring-gear size. The many
upgrade options for the rear axle assembly based on the type of expected use are discussed here.
The 7.5-inch rear has fewer options and is not as strong an axle assembly as the 8.8, so if your V-6
car is going to be used for serious racing activities, or if you just want a wider range of options
available to you, swap out the 7.5 for an 8.8. They’re inexpensive enough and widely available both
new and used to make this a logical alternative.
Ford Racing 8.8 inch Rear axle gear kit
Ford Racing offers a great
selection of ring and pinion
gear sets for the 8.8- and
7.5-inch rear ends. Make sure
you get your gears installed
correctly, or they are noisy.
Axle Ratios
I get asked several times each week, “What gear ratio should I put in my Mustang?“ There is no
single answer for this question, but I have my preferences. Many factors that should play into your
gear choice: the type of use the car receives, whether or not it’s a daily driver, whether or not fuel
mileage is a concern, whether long distance drives are part of the car’s requirements, etc.

Most Mustangs came from the factory with 3.08:1 or 3.27:1 from ’94-’98 and 3.27:1 or 3.55:1 from ’
99-2004. These ratios are mandated in most cases by CAFE regulations and NVH (noise, vibration,
and harshness) concerns – not for optimum performance. Believe me, if the Ford engineers could
give us all 3.73:1 and meet their other objectives, they would.
An everyday Mustang used for a variety of activities works well with 3.73:1 gears and still gets
decent fuel economy. The improvement in acceleration is noticeable, and yet the engine RPM on
the highway is still quite reasonable. This is true for V-6, 5.0L, or 2-valve 4.6L engines. The ’96-
2001 Cobra and the ’03-’04 Mach 1 can benefit even more from a 4.10:1 ratio because of the extra
1,000 rpm available in the 4-valve engine’s operating range.
Ford Racing 8.8 inch Rear Axle Girdle
The axle girdle provides
additional support to the
differential bearing caps,
which can prevent breakage,
particularly on drag cars, with
their extreme shock loads.
Also, since the 4-valve engine does not produce as much torque in the 2,000 to 3,000-rpm range,
the higher ratio gets the engine into its power band more quickly. I wouldn’t hesitate to run more
gear on a daily driver car that does not need to run down the highway at 70 mph all day. A set of
4.30:1 or even a 4.56:1 is not out of the question on a modified Mustang that makes power in the
4,000 to 7,000 rpm range.

As far as the dragstrip goes, a 4.10:1 ratio combined with a 28-inch tall tire is a popular
combination, allowing a trap speed up to about 130 mph at 7,000 rpm. Combinations with higher
RPM or mph potential may require a different ratio. The objective is to reach peak operating RPM in
high gear just as you pass through the timing lights, so the tire size, power output, and RPM limit all
play a role in choosing the best gearing. In road racing, it’s a similar deal. You need to gear the car
for the maximum speed required on the fastest section of the track. Assuming that your Mustang
has a close-ratio fifth gear, a 3.73:1, 4.10:1, or 4.30:1 ratio should be in the ballpark depending on
the track.
Ford Racing 8.8 inch Rear Axle Gear Installation Kit
When installing a new
ring and pinion, or a
new differential, it’s
convenient to
purchase a complete
installation kit like this
Ford Racing package.
Once you’ve selected your gears, you have some other components to consider before you get to
the installation. A word on purchasing ring and pinions: Buy them from Ford Racing. Period. The
Ford ring and pinions are made on the same equipment as their production gears, so they’re
dimensionally the same as the stock gears. This means less trouble for you installing them because
most of the time the original pinion bearing shim can be reused and is correct for backlash and
gear pattern. Whenever someone brings us another brand to install, it inevitably takes longer and
is more difficult to install. Just buy the Ford gears – they save you time and money in the long run.
Differentials
Most performance SN95 Mustangs come with Ford’s Traction-Lok limited-slip differential. It’s
adequate for moderate performance use, but it has a low breakaway torque, limiting the amount of
power that can be applied to both wheels at the same time. Ford actually supplies an additive for
their limited-slip differential that allows smooth slippage between the friction discs and the steel
plates, since most customers are annoyed by differential chatter while cornering in the city. A dyed-
in-the-wool enthusiast trades some degree of noise for the ability to put power to the ground
through both rear tires.
Ford Mustang 8.8 inch Rear Axle Limited Slip Differential
The Ford Traction-Lok
limited slip differential uses
friction plates to limit wheel
spin. Rebuild kits can renew
the differential back to
original performance.
We can increase the friction in the Traction-Lok by modifying the clutch pack. I first learned to do
this back in 1990, when we had to run the original equipment differential in my Firehawk road-race
Mustang. Breakaway torque is the amount of torque that can be applied to one wheel while the
other is held stationary, before the wheel breaks free. This can actually be measured with a torque
wrench and some suitable adapters. A new stock Traction-Lok is in the 80- to 100-ft-lb range. The
factory service manual describes a breakaway torque of 20 ft-lbs as an acceptable service limit for
an 8.8-inch Traction-Lok differential. You might as well not even have a limited slip differential at
that level! The standard Trac-Lok’s clutch pack arrangement has alternating friction plates and
steel plates. Each side of the differential has three clutch plates and four steel plates. We can add
one more friction plate on each side between the two steel plates that come back to back from the
factory. This increases the friction surface area, and also increases the preload on the “S” spring
between the two halves of the differential. We need to be careful we can still get the axle C-clips in,
but as long as we can accomplish this the axle breakaway torque is increased to over 300 ft-lbs.
Now the differential has the ability to transfer a much higher level of power before spinning the
lightly loaded inside tire.
Auburn limited slip differential for Ford Mustang 8.8 Inch Rear Axle
The Auburn limited-slip
differential uses cone-type
clutches instead of plates.
The spring preload
between the two opposing
friction cones determines
the breakaway torque
value. (Photo courtesy
Auburn Gear).
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This has been a sample page from

High Performance Mustang Builders Guide High-Performance Mustang Builder's Guide
1994-2004
by Sean Hyland
High-performance ‘94-‘04 Mustangs represent the high-water
mark for late- model Mustang enthusiasts. From the ’94-’95s with
the 5.0L, through the ‘96-‘04 models with the 2- and 4-valve 4.6
Ls, to the Bullitt, Mach 1, and factory supercharged ’03-‘04
Cobras – never before has such a range of highly modifiable
performance cars been available. These Mustangs were amazing
performers straight from the factory, but they can be even better
with the right combination of performance parts.
Regardless of which ’94-’04 Mustang you start with, the availability
of high- performance parts is unparalleled. You can build your
Mustang for drag racing, road racing, or improved street
performance – and High- Performance Mustang Builder’s Guide
1994-2004 will show you how! Author Sean Hyland uses over 300
photos to explain how to upgrade your Mustang’s engine,
suspension, chassis, transmission, rear end, brakes, and body.
There’s even a special chapter on getting active in various forms
of organized racing.

Sean Hyland is the proprietor of Sean Hyland Motorsport, which
builds and supports internationally competitive Mustangs for road
racing, drag racing, and everything in between. Sean recently did
a complete Mustang build-up for Speed Channel’s Sports Car
Revolution and is also the author of the bestselling title How to
Build Max-Performance 4.6-Liter Ford Engines.
Click below to view sample
pages from each chapter.
Chap. 1 - Chassis
Chap. 2 -
Wheels and Tires
Chap. 3 -
Suspension
Chap. 4 -
Brakes
Chap. 5 -
3.8 Engines
Chap. 6 -
4.6 Modular Engines
Chap. 7 -
Transmissions
Chap. 8 -
Rear Axles
Chap. 9 -
Aerodynamics
Chap. 10 -
Safety Equipment
Chap. 11 -
Get Involved!
Chap. 12 -
Project Cars
8-1/2 x 11"
Sftbd.
144 pgs.
300+ B/W photos
Item: SA106P
Price: $22.95
Click here to buy now!
This is a great book
that any Mustang
enthusiast will enjoy!


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How to Build Performance 4.6 Liter Ford Engines
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Building 4.6 & 5.4 Ford Horsepower on the Dyno
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