BCM Forscan change for tire size

[CTX148]

Quick question for someone who has a performace pack Mustang with 275/40r19 rear tires. Can someone check their factory As Built data for the following address:
726-15-02. **xx xx--

I'm using Rocket's calculator and determined the first two characters should be (5,7) based on the calculator.

However the calculator shows I should have had (3,7) as the factory As Built data, and I actually had (3,8). Was just curious why the difference between factory As Built data and calculator and wanted to confirm.

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[GrabThatBlue]

I'm going to work on my car tomorrow morning and will look for you.
And maybe somebody already has these data ready to share with you before tomorrow.

 

[CTX148]

Thanks I appreciate it!

 

[Sainty]

Here you go!

FORScan backup from vehicle: 726GFG28259405F

From https://www.motorcraftservice.com/AsBuilt

<DATA LABEL="726-15-02">
  <CODE>8259</CODE>
  <CODE>405F</CODE>
  <CODE/>
</DATA>

 

[CTX148]

Thanks for everyone's help, tried the input from the calculator (5,7) and noticed the car was actually 2 mph slower than indicated on the speedo.

Then I used the input from Sainty above (8,2) which is the factory setting for the PP wheels. Now the car was only travelling 1 mph slower per the GPS than indicated on the speedo.

Finally I reverted back to the original As-Built setting of (3,8) and it too has the car going 1 mph slower than the speedo shows.

So for those Ecoboost owners that end up upgrading to the 275/40r19 size PP wheels, it turns out there's not much of a difference in the BCM settings. At least up to normal highway speeds.

(Original tire size was 235/50r18 for ref.)

 

[roket]

in the interest of transparency, here's the code used for the calculator

function tirecalc() {
    //calculate the tire circumference, multiply it by 0.967, then convert it to hex
    var tirewidth = document.getElementById("tiresize_width").value;
    var tireaspect = document.getElementById("tiresize_aspect").value / 100; //convert the aspect ratio into a tenths
    var wheeldiam = document.getElementById("tiresize_wdiam").value;
    var tireheight = (tirewidth * tireaspect) * 2 + (wheeldiam * 25.4); //multiply the width by the aspect ratio then add that to the wheel diameter converted to mm
    var tirecirc = Math.floor(tireheight * Math.PI); //mutliply the total height by pi to get the circumference
    //console.log(tirecirc);
    var finalcirc = Math.ceil(tirecirc * 0.967).toString().padStart(4,0); //multiply the circumference by 0.967 because Ford does that for some reason, then round that up, convert it to a string, and pad the start
    //console.log(finalcirc);
    var hexcirc = parseInt(finalcirc).toString(16).padStart(4,0).toUpperCase(); //convert the new circumference to an int, then convert it to base16, pad it, and make it uppercase
    document.getElementById("tiresize_result").innerHTML = hexcirc;
    document.getElementById("tiresize_result_mm1").innerHTML = tirecirc;
    document.getElementById("tiresize_result_mm2").innerHTML = tirecirc;
    document.getElementById("tiresize_result_n1").innerHTML = hexcirc.slice(0,2) + "XX";
    document.getElementById("tiresize_result_n2").innerHTML = hexcirc.slice(2,4) + "XX";
}

what it does is it gets a bunch of stuff from the HTML that was in the inputs, then does the following:
calculates the full height of the tire in mm
calculates the circumference in mm, rounded down
finds the circumference used for the hexadecimal by multiplying the circumference just found by 0.967, then converts it to a string so that i could add leading zeros if it was less than 4 characters in length
finally, it assumes the variable we just did was a whole number, and converts it to a base-16 string, then pads it with leading zeros again if needed, and finally makes the whole thing uppercase

the thing is, this is based on some stuff i found at the time (August 2023) which i interpreted to mean Ford for some reason uses the circumference times 0.967. thanks to this thread, sometime very soon i will be updating the calculator. it remains to be seen if the hexadecimal part stays or it just calculates circumference, but i will also work on adding support for non-metric tires

 

[Will2]

Thanks for everyone's help, tried the input from the calculator (5,7) and noticed the car was actually 2 mph slower than indicated on the speedo.

Then I used the input from Sainty above (8,2) which is the factory setting for the PP wheels. Now the car was only travelling 1 mph slower per the GPS than indicated on the speedo.

Finally I reverted back to the original As-Built setting of (3,8) and it too has the car going 1 mph slower than the speedo shows.

So for those Ecoboost owners that end up upgrading to the 275/40r19 size PP wheels, it turns out there's not much of a difference in the BCM settings. At least up to normal highway speeds.

(Original tire size was 235/50r18 for ref.)

To calculate the correct number, you have to convert the tire circumference (in millimeters) to hex (hexadecimal). The OEM values don't quite line up with any calculator I've used online, and different than what one would calculate using the advertised specs from even the tire manufacturer webpages I've found (Ford's values are slightly smaller).

I don't know for a Performance Pack wheel set, but for:

Ecoboost base 18" wheels (235/50/18 tire):
726-15-01: ---- ---- --08
726-15-02: ---- ---- 38--
Together, those 4 digits make up "0838" which is hex for 2104mm

GT base 18" wheels (255/45/18 tire):
726-15-01: ---- ---- --08
726-15-02: ---- ---- 27--
Together, those 4 digits make up "0827" which is hex for 2087mm

Ecoboost Nite Pony Package, Wheel & Stripe Package, Bronze Appearance Package, Premier Polished Aluminum Wheels (255/40/19 tires):
I believe Ford also uses 2087mm, so same as GT base wheels above.

 

[CTX148]

Thanks Rocket, I appreciate you creating the calculator in the first place! Your posts have been really helpful in me learning to use Forscan. I wasn't trying to be critical at all, was more like me trying to make sure I was understanding the tool and inputing the right values LOL.

 

[roket]

Thanks Rocket, I appreciate you creating the calculator in the first place! Your posts have been really helpful in me learning to use Forscan. I wasn't trying to be critical at all, was more like me trying to make sure I was understanding the tool and inputing the right values LOL.

i really needed to update it anyways. it looks like the consensus has changed from the right value from "0.967" to "0.968". i also really needed to add an imperial tire size feature. so i did, it should be going up soon-ish. i also went ahead and added a notice for certain tire sizes based on what i saw online people were hitting limits at. so really, thank you. without your comments, i might have just kept putting off the update

 

[npole]

I don't think there's a fixed value to multiply to. In example for our EU DH (275/40 R19) it would be 0,987:

2207 (real circumference) * 0,987 = 2178 (0882 HEX)

PS: do you know you can do all that by using ChatGPT using a "natural" language?

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