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Speedometer Calibration

So you changed tires and now your speedometer is off?

  1. Find out how far off your speedo is (with the new tires installed)
    1. Using Milemarkers
      1. Drive on the highway for 10 miles (according to mile markers) and note how far your odometer advances. Divide the odo reading by 10 and that will tell you what percentage your speedo is off.
        Example: If you drive 10 miles and your odometer advances 9.4 miles, 9.4 / 10 = 0.94
        (So your speedometer is reading about ~6% slow).
    2. Using GPS
      1. Drive at your favorite cruising speed on the highway (using the speedo), and note the gps indicated speed. Divide the speedo speed by the gps speed and that will tell you what percentage your speedo is off.
        Example: If your speedo reads 65mph and the gps reads 69.1mph, 65 / 69.1 = 0.94
  2. Find out what speedo gear you have.
    1. Remove the bolt(1) holding the speedo cable to the transfer case.
    2. Pull the cable w/gear out of the transfer case.
    3. Count the number of teeth, and note the color for verification
  3. Find out which speedo gear you need.
    1. Multiply the number of teeth by how far off your speedo is and this will give you the gear you need.
      Example: If you find that you have an 18 tooth gear and your speedometer is .94 off(from above), 18 * .94 = 16.92
      Since they don't make a gear with 16.92 teeth, you'll need a 17 tooth gear which is the closest.
  4. Install the correct gear.
    1. Pull the metal clip off the old gear and the gear should come right off the cable.
    2. Install the new gear and reinstall the metal clip.
    3. Insert it back into the transfer case and install the retension bolt.

References (1966-1977)

Part Number # of teeth Gear Color
C0DZ-17271-A 16 teeth wine
C3DZ-17271-C 17 teeth white
C0DD-17271-B 18 teeth gold or yellow
C0DZ-17271-B 19 teeth natural color
C1DZ-17271-A 20 teeth black
C4OZ-17271-A 21 teeth red

Gear Sources

Attributions

Part Numbers and MileMarker Calculations from Marc Reiter
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