How to recal for larger tires

My speedo/odo is about 7% low with these 35" tires. So how does one
go about adjusting the thing to make it accurate?

Steve

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.

Thats great info. I will be using this when I put on my 38's.

Yes, thank you. I'm looking forward to trying this.......as soon as it gets out of the shop from getting a DP!!

Doesn't it make a difference what gearing you have? If so, how do you calibrate for that? I will be changing from 4.10's to 4.30's when I do my whole swap.

Crap. Good question!

o i never really thought of that

yep gearing matters

i used this method when i first put my 38s on and it was perfect but since i put my gears in i cant figure out the right math...

you only get six reprog's before the cluster is junk, so becareful (im on my last attempt)

if any one is real good with math maybe they can com up with a formula ?

Are you sure that the rear end gearing has anything to do with what the spedo reads? I thought the recal only told the pcm how far the tire rotates for a given amount of movement by the VSS unit on the rear end. Or I could be wrong on that. I need to do the same thing to mine as I went a little shorter than stock tire to tweak my RPM's for the road speed I wanted to maintain with max torque. So if anyone else knows more than we do please enlighten us:bow:

I dont think gearing makes a differenece, because dont our trucks get the wheel speed signal from a magnetic pickup that is attached to the rear carrier, separate from the ring gear.

I dont think gearing makes a differenece, because dont our trucks get the wheel speed signal from a magnetic pickup that is attached to the rear carrier, separate from the ring gear.


nope, the tone ring is attached to the carrier and so is the ring gear, so the carrier spins with the ring gear

wait, i think you just turned the light on.....gearing shouldnt effect our trucks. even though the driveshaft spins at a different speed one rotation or the ring gear is still one rotation of the tires so since, like you said, our trucks get there signal from the tone wheel on the carrier(wich is wheel speed) it should not effect the speedo.....

does that sound right? I tend to over think things.....

well now ive confused myself, some one tell me im right or wrong

wait, i think you just turned the light on.....gearing shouldnt effect our trucks. even though the driveshaft spins at a different speed one rotation or the ring gear is still one rotation of the tires so since, like you said, our trucks get there signal from the tone wheel on the carrier(wich is wheel speed) it should not effect the speedo.....

does that sound right? I tend to over think things.....

well now ive confused myself, some one tell me im right or wrong

I think the truck gets the signal for the speedo from the sensor mounted on the transmission. The one in the rear housing is mostly for the ABS system. I got this info from someone having problems after swapping a used tranny into their truck and the speedo was way off afterward. That was a 4R100 though so our trucks with the E4OD may be different, but I don't think they are.

e4ods dont have a speed sensor in the tranny the only wiring to them is for the vavle body

on mine there was no speed sensor on the tranny, if there was i would have had a problem when i did the zf5 swap. the only wires on my zf are the neutral safety and the reverse switch. The abs system could be tied into the vss, if the vss is on the rear end, gear ratio would not make a difference. A 38" tire will spin the same speed at 60mph regardless of gear ratio. the driveline speed would be different. i will be moving down to 3.73's from 4.10's and up to 37's or 38's from 35's. we'll see if it works, just my .02:blah:

Sounds like I opened up a can of worms. Unfortunately I have no idea to the answer so, yeah.