Speedo is wrong HELP
Senior User
Joined: Nov 2006
Posts: 241
Likes: 0
From: Southern California
FWIW, the % of tire diameter you went, simply subtract that percent from miles traveled (indicated) and you have a good indication of how many miles you've traveled (actual)... same goes for mph... for example, you went from a 30" tall tire to a 33", thats a 10% difference (3/30 or .1)... say you traveled 1000 miles on your dash, simply take out 10%, or 100 miles... meaning you traveled 900 (actual) miles instead of the indicated 1000 miles... clear as mud? 
... same goes for mph...
Joey
... same goes for mph...Joey
FTE Leadership Emeritus
Joined: Sep 2000
Posts: 23,174
Likes: 1,678
From: Stephensville WI
Everyone's speedometer is off to some degree. A 5% (which is miniscule) error is still +/- 3 MPH at 60 MPH. Yours is a little more off, but not much.
You could get it calibrated perfectly, but in a few months it would be off again due to tire wear.
You could get it calibrated perfectly, but in a few months it would be off again due to tire wear.
Elder User
Joined: Jan 2003
Posts: 898
Likes: 0
OK... here are the instructions:
Below is the way to measure your tire circumference and recalibrate your speedo....This info is from Baz's site:
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.
--------------------
Below is the way to measure your tire circumference and recalibrate your speedo....This info is from Baz's site:
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.
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