Originally Posted by FIRST SET

Recalibrating odometer/speedometer

To recalibrate your speedometer after changing tire sizes:

Start with 20186, divide by actual tire size in inches. Divide again by 66.66.

Example: 20186/34.5=585.10/66.66=8.78

Thus for 34.5 inch tires, the calibration is 878.

To set the calibration, ground the single wire connector under the glove, hold in reset button on trip meter and turn ignition to ON. From here, use Select to get proper calibration and press Reset to store.

Warning: You can only change the calibration 6 times without replacing the module. Your Ford dealer should be able to do this for you too if you aren't comfortable doing it yourself

Originally Posted by SECOND SET

My truck is a 97 and the calibration procedure worked for me. I have Michelin 235/85/16 tires. First I took a red felt tip and marked about a ½” high section of the sidewall tread where it touched the ground continued the mark onto the pavement. I then had my wife slowly move the truck forward and marked exactly on the pavement where the mark touched. I did this for three revolutions to get a good average. From the first mark to the last was 290 3/16” 290 3/16” = 290.1875” per 3 revolutions of the tire in inches. Divide that by 3 to get a circumference of 96.72916”. Circumference = pi X diameter. I did all this to get an accurate measure of the circumference so that I could use the formula below… Remember this measurement is only for my tires that have 30,000 + miles on them, your 295/75/16 will be different. You need to have an accurate diameter reading to use the speedometer calibration formula. So…if C=pi X d then C/pi = d….or 96.722916/3.1416 = diameter. Diameter is 30.8” rounded off to the nearest tenth.

Using the formula to find the correct code to enter 20186/diameter = a number, in my case 655.4. Divide your result by 66.66 to get the final number (9.831 in my case) with a diameter of 30.8 inches my code to enter into the speedometer is 983 (drop the decimal and only use the 983) You will need your calibration code for your tire size before you start the actual calibration To set the calibration, ground the single wire connector under the glove box. ”Yes there is only one and it has a spade type plug on the end". I used a wire with clips on both ends to ground the plug to a screw under the dash. Look at your speedometer on the face there should be a "reset" button and a "select" button. While holding the reset button in on the trip meter, turn the ignition to 'on'(wire still grounded). Let go of the reset button. The speedometer display will sweep and the existing code will be displayed. To enter your new code, push on the "select" button to get to the proper calibration code. You may have to push “select” a whole bunch of times until you get to your code. If you turn off the key at this point the original code will be reset. To store your new code press “Reset”. Turn off the key. Unplug the ground wire.