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
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
The prefered method is to use the manufacturer tire info of revs per mile.
There is a formula to get the conversion constant using the Rev/Mile number
Conversion Constant = (Tire SAE avg Revs/mile x number of excitor ring teeth) / 8000
10.25 has 120 excitor ring teeth
Simplifying the above equation...
This would be the preferred method. Tire rev's per mile / 66.66 = conversion constant
The 66.66 is 120/8000, thus simplifying the equation for a 10.25/10.5 rear axle.
Also to simplify the equation that Brian put up....
For a 120 tooth exciter ring
30279/tire diameter = conversion constant
I got the 30279 by simplifying the consants... 20186/.6666
Does this work regardless of your differential? example does a 3.55 and a 4.10 use the same formula?
I have the 4.10 and I did this calculation.
My speedo is closer but I am still off at 65 mhp (62mph on gps vs 65 on truck speedo, was show just north of 70 mph on speedo before adjustment) or is this as close as I should expect?
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