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Gears will increase acceleration but decrease top speed and fuel economy. Gear ratio is often associated with the differential in your car. The internal combustion engine does not make enough torque to move a car (at any kind of speed) so gearing is used to multiply torque output in transmissions and final drives. Check the sticker on the driver side door for rear axle codes. Here is the list of codes and what they mean:

Under the "AX" on the door jam sticker:

  • As a guide in changing rear end ratios, use the following formula.

    [new gear ratio/old gear ratio] = RPM factor increase(or decrease)for a given speed.

For example should you have 2.05:1 gears now and are turning 2,000 RPM at 65 mph in high gear now; and you change to 4.10:1 gears, then you will be turning 4,000 RPM at 65 mph with the new gears for a factor of 2X for the change.

If you go from your stock 3.27 to say 3.73 you will increase your RPM at any given speed by a factor of 1.1406X or an increase from say 3,000 rpm in OD at 70 mph to 3422 rpm at the same speed. From 3.27 to 4.10 = increase of 1.254X or 3,762 rpm for the same speed.

Which speedo gear to put in?

The 21 tooth is purple, part #D0OZ-17271-B, for an automatic trans. I think you need the 21 tooth gear that is not sold by Ford, but many Mustang shops sell it. Be warned, the teeth are small and the gear wears out fast. Look in the FAQ section of the SCCoA site ( for more info on speedo gears and speedometer calibration units.
If your AODE trans has an 8 tooth drive gear, then the above calculations are correct. But if it has a 7 tooth drive gear, then the calculations would be different. If you pull the stock gear first and count the teeth or determine the color and you know your stock rear gear ratio, you can use the SCCoA gear calculator to determine what drive gear you actually have. Then you will know which driven gear to buy. The drive gear is internal to the trans and not easily changed. The driven gear is the one on the end of the speed sensor and is the one that is usually replaced when rear gears are changed.

Car Teeth Color Part #
Auto 18 Grey C7SZ-17271-B​
Auto 19 Tan C7VY-17271-A​
Auto 20 Orange C8SZ-17271-B​
Auto 21 Purple D0OZ-17271-B​
Manual 19 Pink C0DZ-17271-B​
Manual 20 Black C1DZ-17271-A​
Manual 21 Red C4OZ-17271-A​

Note: The 94-up (with the AODE) use a 7 tooth.
Since this question comes up all the time, I decided to hack up a quick spreadsheet that shows the relationships here. Across the top are the available 8.8" axle ratios, down the side are the ratios in the stock T-5 tranny for each gear. The "factor", as I call it, is a simple constant for each axle ratio and each gear that, when multiplied by the engine speed in RPM, yields the speed in MPH. As an example, 4th gear with 3.27s at 5000 rpm is 5000*(.02238)=111.90 MPH. This is a useful set of numbers for those of us with 85mph speedos who sometimes play at high speeds.

I also included the RPM number for highway cruising at 65 in 5th, just so that you could have an idea of how high the motor would be buzzing. One other thing- this table is based upon a tire diameter of 24.6 inches, which just happened to be the diameter of the Yokohama A008Rs that I used to run. These numbers will obviously vary based upon the diameter of your chosen tires.

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