How to RECALIBRATE your SPEEDOMETER
HOW TO RECALIBRATE FORD TRUCK SPEEDOMETERS
To recalibrate your speedometer after changing tire sizes:
Start with 20186, divide by actual tire size in inches. Divide again by 66.66.
20186 / 34.5 = 585.10
585.10 / 66.66 = 8.78
Thus for 34.5" tires, the calibration number is 878.
To set the calibration, ground the single wire connector under the glove box (often Lt. Blue wire w/ a Yellow tracer, but you might want to check a manual for specifics), hold in Reset button on trip meter and turn ignition to ON. After the speedometer needle cycles from zero to top speed and back to zero, press Reset button again. You should now see the calibration # that you currently have. From here, use Select to get proper calibration, and press Reset to store.
When you press Select, the number only goes down, but don't worry if you need a higher number. After you reach a certain number (400 or 500 I believe), it will start back at the highest number the speedo can be calibrated with (often 1100), and go down again.
Larger tires will require a smaller calibration number.
If your new calibration number needs to be larger than the highest number allowed, just go with the highest number allowed.
If you shut your ignition switch OFF before you press Reset to store the new calibration number, the number will remain what it was before you started.
With stock 235-75 15's, the calibration number should be about 1044, but you may find that it's been set at 972 from the factory. This means that your odometer has more miles on it than you have truthfully traveled.
You can only change the calibration number 6 times without replacing the module.
I have no idea what years this applies to, but it has been suggested that if you have a digital odometer, it applies to you. The easiest way to see is to hold the Reset button down while you turn the ignition switch to the ON position. If the speedometer needle cycles, this applies to you.
Last edited by PKRWUD; 05-08-2002 at 03:01 AM.