Is there a way to recalibrate my speedometer? I got bigger tires than stock and compared to others my speedometer seems to say 5mph faster than I am going. Any help is appreciated

 

From Steve Baz 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.

 

what size you running now John?

 

im still runnin the old 265 75 R16 wranglers but when i cruise with people it says im doin 60 and they are goin 55

 

Thanks alot tj. I'm gonna do that as soon as I can

 

use a gps to see your exact speed before you trust someone else's speedo, you only get 6 attempts then you need a new speedo.

 

who were you doing it with? test it on one of the radar carts that says what your speed is

what rpms are you running at 60? at 70? should be 1500 and just shy of 2000

 

someone must have all ready recalibrated it for 285 75's mine was right about 5mph off with 265's threw some 285's on there and its bang on now, actually just a touch bit slow, using road markers over 100 miles im 3 tents of a mile off....lol

 

I think OE most vehicles read about 3 miles faster than they are really going. Its a conspiracy!

 

Ok a recap. I did what you said TJ and I ended up with 20 as my circumfirence??? I measure the diameter and get 30 times 3.14 and get 94. times that by 3 and get 282. I measured over 3 oil spots to. I'm totally lost now. Should I have 4 dots on the ground?

 

hold off before you do it, i will come give you a hand

 

ok i will call you this weekend or somethin. i'll come to your house if you dont mind because me and dad moved so we live farther away

 

hmmm, where did you guys move to?

 

in fair hill now. my grandma aint doin good so were takin care of her. if you are going towards newark i live on middle road before wesley's. i live in the developement called middlecroft. its the second one on the right