Resetting Recalibrating your PSOM for the Correct Speed

    Ford uses a module called the PSOM to hold your speed calibration information as well as your odometer memory. Pressing the select which the truck on will change the display from your actual mile portion to the trip meter and back. When displaying the trip meter you can reset it by holding down the reset (duh!). If you hold the Reset button while turning the key forward to Run, and then releasing the Reset, you will see your speed ratio constant or conversion constant. The needle will do a full sweep test, and the odometer display will show "E ## #", meaning English units, the microprocessor revision and the conversion constant lockout count. Pressing Reset again shows "###CAL", which is the conversion constant (without a decimal).

    If you want to change the constant, find the single-wire connector under the glove box and ground the LB-Y wire before turning the key on (you still have to hold the Reset, as before). Everything is the same as before, except "CAL" will flash, indicating that the constant can now be changed. Press the Select button to decrease the constant by 0.01, each time. When the constant reaches 5.00, it goes back to 11.00 . Larger tires or fewer teeth on the tone ring require a lower constant - R&P changes have no effect. When you have the constant you want, press the Reset button to lock it in. THIS WILL DECREASE THE LOCKOUT COUNT BY 1 - WHEN IT REACHES 0, NO FURTHER CHANGES CAN BE MADE. To abort the changes and maintain the lockout count, turn the key off before pressing Reset. When finished, turn the key off and disconnect the LB-Y wire from ground.

The formula for finding the constant for your tire size are located HERE.

This yields the Following Chart: 

Note tire size does not take in consideration actual tire size not does it take under consideration tires squatting under the load of the truck.  Ford allowed 1.33 inches for the stock 31 inch tire for such.  Thus the stock constant is 9.18 and is for a tire size of 29.67 inches.

Resetting procedure provided by Steve.  Formulas received direct from the manufacturer of the PSOM