Spedometer 10% too fast
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Spedometer 10% too fast
with smaller tires on my truck, the spedometer is 10% faster than what it should be. the truck originally had 35"s and is there a way to reset the spedometer? and did it cause the miles to go up faster with smaller tires?
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Joined: Aug 2006
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From a post from BlueMule
First, you'll need to know the actual height of the tires, not the
nominal height of 35" that's printed on the sidewall. To get that,
make a mark on the edge of the tire at the ground, and mark the ground
directly under the mark. Now, roll the truck far enough for the tire
to have made three revolutions. When your mark on the tire is at the
very bottom, near the ground, mark the ground again, and measure the
distance between the two marks on the ground. Divide that distance by
three, since there were three revolutions of the tire, then divide
that number, (which is the circumference of your tire,) by 3.14. The
answer you have is the true height of your tire. Write that number
down, because you'll need it for the next step. Divide 30282 by your
actual height. If your true height is 34", the math goes something
like this: 30282/34=891. That would be your speedometer calibration
code."
"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."
__________________
Rodney
First, you'll need to know the actual height of the tires, not the
nominal height of 35" that's printed on the sidewall. To get that,
make a mark on the edge of the tire at the ground, and mark the ground
directly under the mark. Now, roll the truck far enough for the tire
to have made three revolutions. When your mark on the tire is at the
very bottom, near the ground, mark the ground again, and measure the
distance between the two marks on the ground. Divide that distance by
three, since there were three revolutions of the tire, then divide
that number, (which is the circumference of your tire,) by 3.14. The
answer you have is the true height of your tire. Write that number
down, because you'll need it for the next step. Divide 30282 by your
actual height. If your true height is 34", the math goes something
like this: 30282/34=891. That would be your speedometer calibration
code."
"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."
__________________
Rodney
Postmaster
Joined: Feb 2002
Posts: 2,773
Likes: 0
From: Bonney Lake, WA
To quote Steve Baz:
This is just another way to say the same thing.
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.
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.
