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Mustang Speedometer Gear Calculator

Posted 8/6/2013 by Jonathan McDonald

Get the right speedometer gear for your 1979-1998 Mustang based off of rear gear ratio and tire size! No more guessing with this Mustang Speedometer Gear Calculator!

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Mustang Speedometer Gear Calculator

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

Options:

  • 6 Tooth
    • Select this if you already have, or plan to, change your T5 transmission to a 6 tooth drive 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
      • 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 off 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 best way to correct the speedometer on a 1996-1998 Mustang is by using a Speed-Cal or the Ford Racing Speed Dial
  • The only way to correct the speedometer on a 1999-2010 Mustang is via a Speed-Cal, Ford Racing Speed Dial, or an aftermarket Tuner (Such as SCT)
  • The only way to correct the speedometer on a 2011-2014 Mustang is via an aftermarket Tuner (Such as SCT)