1999-2003 Rear End; Performance Modifications; Gears - Factory Five Racing Mk4 Roadster Assembly Manual

1999-2003 rear end

Because of the width of this rear end, custom wheels are needed if 10" or wider wheels are wanted. Use
two (2) 9" wide, 5.95" backspace wheels in the front. See the table above for different width wheel
backspacing.

Performance Modifications

The following modifications and set-ups fall under the category of getting more horsepower from your
engine and making your car perform better. You will find a hundred people a day that will tell you what
you've got to have. We just want to start you out with the easy modifications that net large returns
without large cash outlays or serious changes to the stock motor and drive train. Remember that a 2,100
lb. car stresses the parts less than a 3,100 lb. Mustang does, so all of the components will last longer and
can take more abuse than they did on the Mustang. Check out www.factoryfive.com to see what we
offer.

Gears

The Mustang 5.0 manual transmission car comes with either 2.73 or 3.08 rear end gear ratios from the
factory. If you can get a rear from an automatic car it will have 3.27 gears. There is an ID tag on the
rear differential. If you find that your rear end is missing its ID tag, there is a good chance that the rear
end was modified. To find out what ratio is on the differential, mark the pinion and axle at a start point.
Turn the axle exactly one rotation and count the number of rotations that the pinion makes compared to
the reference mark. For example: 3 ½ turns is a 3.55 rear ratio, etc.... The other way to check the ratio
out is to open the rear differential up and look on the ring gear. While the rear is open change the fluid.
This never gets done enough on the car.
Changing the gear ratio is one of the least expensive ways to improve your car's performance and it is
easiest to do before assembly, since the rear is out of the car. If you know you are going to change the
gears get the 2.73 rear since these are the most readily available, the least expensive and the least
abused. All of the gear ratios work well with our car but we've found that installing an aftermarket 3.55
ratio really wakes the motor up. If you change the rear end gears, don't forget to change the
speedometer sending unit gear so that the speedometer reads correctly. Wherever you get the rear gears
from should be able to get the correct gear for your application. If you cannot find the correct gear call
"Texas Mustang Parts" 800-527-1588, www.texasmustang.com , they have the "hard to find" gears.
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www.factoryfive.com 508-291-3443