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Mustang Speedometer Gear Calculator

Created by Taylor Ward
Last Updated 4/26/2022

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 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)

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    About the Video

    How To Calculate Speedometer Gear For Fox Body & SN95 Mustang

    Check out our YouTube channel for even more tech tips, installation videos, how-tos, and more. The best place to go for anything Mustang related!

    Published on 2020-05-08
    Speedometer Gear Calculator: https://latemodelres.to/GearCalculator_YouTube
    Shop 79-04 Speedometer Gears: https://latemodelres.to/SpeedometerGears

    Follow along as Landan shows you how to calculate your speedometer gears and install them in your Fox Body or SN95 Mustang

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    #LMR #Foxbody #SN95

    Due to factors beyond the control of LMR.com, we cannot guarantee against the improper use or unauthorized modifications of this information. LMR.com assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. Use this information at your own risk. LMR.com recommends safe practices when working on vehicles and or with tools seen or implied in this video. Due to factors beyond the control of LMR.com, no information contained in this video shall create any expressed or implied warranty or guarantee of any particular result. Any injury, damage, or loss that may result from improper use of these tools, equipment, or from the information contained in this video is the sole responsibility of the user and not LMR.com.

    Mustang Speedometer Gear Calculator Tech Info