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Mustang Speedometer Gear Calculator

Created by Taylor Ward
Last Updated 8/5/2021

Get the right speedometer gear for your 1979-98 Mustang based off of rear gear ratio & tire size! No guessing with this Mustang Speedometer Gear Calculator!

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Mustang Speedometer Gear Calculator

 * see step 2 below

How to use the Mustang Speedometer Gear Calculator

Step 1 - Select Transmission Type

This is pretty straightforward, select "Manual" for a manual transmission or "Automatic" for an automatic transmission.

Step 2 - Select Speedometer Drive Gear Teeth

  • 6 Tooth
    • Select this if you have a:
      • T5 transmission that has been converted to a 6 tooth gear to correct the speedometer more accurately. The black 6 tooth internal drive gear was originally used in the 4-speed transmission, but it works great to exactly correct for 4.10s when used with a pink 19 tooth driven gear.
    • 7 Tooth
      • Select this if you have a:
        • Factory 1983-1989 Mustang T5 or AOD
        • Ford Racing M-7003-Z World Class T5
        • Tremec 3550, TKO, TKO 500, TKO 600, TKX
        • Tremec T56 Magnum
        • 1996-1998 Cobra T45
      • Or you are converting your 1990-1995 Mustang T5 to a 7 tooth drive gear
  • 8 Tooth
    • Select this if you have a:
      • Factory 1990-1995 Mustang T5, AOD, AODE
      • 1996-1998 GT T45, AODE, 4R70W
      • 1994-1998 V6 T5, AODE, 4R70W

    Step 3 - Select Rear End Axle Gear Ratio

    Mustang rear gears from 2.73:1 all the way to 4.56:1, pick the ratio you currently have or are planning to install.

    Step 4 - Select Tire Width

    Your tire width is measured in mm. For example, if your tire size is 275/40/17, your tire width would be 275.

    Step 5 - Select Tire Series

    Your tire series, or aspect ratio, is measured in a percentage based on tire width. For example, if your tire size is 275/40/17, your tire series (aspect ratio) would be 40. This means your tire sidewall is 40% as tall as the tread is wide.

    Step 6 - Select Rim Diameter

    Your rim diameter is measured in inches. For example, if your tire size is 275/40/17, your rim diameter would be 17.

    Step 7 - Click "Calculate Speedo Gear" Button

    The Mustang Speedometer Gear that most closely matches your inputs will display on the right of the screen next to the calculator!

    A Few Considerations

    • You cannot change the output shaft mounted drive gear in an Automatic Transmission as the gear is part of the output shaft.
    • Do not use a 23 tooth speedometer driven gear on a 1996-1998 Mustang GT or Cobra T45, AODE, or 4R70W. It will self destruct in a matter of feet.
    • The only way to correct the speedometer on a 1999-2010 Mustang is via a Speed-Cal, or an aftermarket Tuner (Such as SCT)
    • The only way to correct the speedometer on a 2011 and newer Mustang is via an aftermarket Tuner (Such as SCT)

    Mustang Speedometer Gear Calculator Tech Info