I have put 315/75/16 tires on my truck. Stock was 245/75/16. That's a 34.5-30.6=4.2.

Being these trucks have an electronic odometer, is it just as simple as a tune change to fix the speedo? Thanks

 

Recalibrate Your Speedometer After Changing Tire Size

There is no tire size/code chart because tire size vs. actual tire dimensions are different between manufacturers. Even between the same tire size and same maker there will be a difference in actual size between the different tread designs.

The Ford calibration equation is:
20186 / tire height in inches = some number, then: some number /.6666 = program code

Example:
Start with 20186, divide by actual tire size in inches. Divide again by .6666

You want to use a rolling measurement to get your actual tire height (If you try to measure the tire in the driveway, use conversion charts, or anything else you will end up with an inaccurate reading again). Use your rear tire since the sensor is located there. Mark the ground and your tire and roll forward 3 times marking the ground each time. (I ran over a spot of oil at a local gas station and measured between my tread mark prints), then divide that number by three to get the average length of one revolution (this will protect yourself against a bad reading using just one revolution). You can then divide the length of one revolution by 3.14 to get actual tire height. Be careful in your measurements, a small amount will change the code. EX: 31.69" tire height is code 956, and 31.21" tire height is code 970.

Now, to program it into your truck:

Warning: You can only change the calibration 6 times without replacing the module.

To set the calibration, ground the single wire connector under the glove box it says PSOM (Programable Speedometer Opdometer Module). ”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 or try a test light and use the clip end to the PSOM and the other jam in a dooe hinge. Now look at your speedometer on the face of your dash while sitting in the drivers seat and 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' while the wire is still grounded. Let go of the reset button. The speedometer display will sweep once and will show a code of some kind then push in again the "reset" button and the existing code will be displayed with RECAL?, mine said 976 RECAL? Now you enter your new code you came up with by dividing the rolling measurement by PI or 3.14 by pushing the "select" button until it gets to the new calibration code number. 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 not be changed or any of the 6 lives will not be used. To store your new code press “Reset”. Turn off the key. Unplug the ground wire. To check the new calibration, use the mile markers on the interstate. Run an indicated 60 mph and it should take exactly 60 seconds to go 1 mile.

 

Sweet... even better no money to spend.... I will try this thank you!

 

Well i just went and tried this, first time i have had time to do so.

The gauge sweeps across like you said, and then when i press the reset button again, it says "970 CAL" when i press the select button nothing happens at all. Am i suppost to hold Select? Im trying to put in 910

Being that it says CAL instead of RECAL would that mean my module has been used up and I need another?

Thanks

 

I just recalibrated mine and had the same problem at first. Double check your ground and try again. Worked for me.

 

Yeah I never posted back, but when I ran a better ground, it workd...