Setting arbitrary tire sizes for speedometer calibration

[RobSMITH3]

And on your first drive speed was changed, it didn't gradually increase ?
I can't get mine to change at all. I think I might try a small increase and see.
Thanks for your time.
I might be back to whine in your ear

[qq1105]

bcm：726-12-01-XXXX-0832-XXXX

[RobSMITH3]

I have the line written in the BCMii, but it makes no diffence.
Apart from DTC codes.
I've 1. Taken the battery off overnight.
2. Reset the PCM via FORSCAN.
3. Reset transmission values via FORSCAN.
NOTHING CHANGES.I'm still 10% under actual.
I'm sure I'm missing something simple.
Any ideas gratefully received.

[k.toddknight]

Recently changed tires sizes on my 2018 F150 Lariat 4x4. Went from 265/70 R18 (09AC, 2476mm) to 275/70 R18 and the speedo was roughly 2mph off, in addition to my DTE calculations. First i looked at the spreadsheet of the tire sizes that others had used, and found where 2 people had figured out that the correct numbers for that size tire was a xxxx-09E7-xxxx (2535mm). I programmed that in (I did get the DTC U0422) and drove it around and tested it, only to find out that the numbers were still off. So i then looked at the google docs calculator and figured out the size manually (09EF/2559mm), with the calculator (09E7,2535mm), and with the GPS (09E1, 2529mm). I first tried the 09EF, still got the DTC, but the speed was still off. Next i tried 09EE (3542mm) (which threw 2 DTC's- UO422 and one related to power steering), then i went down to 09EB (2539mm), which did the same. Both were still off slightly on my speedo. So then I went down below the calculator recommended size of 2535mm and tried the GPS recommended size of 2529mm, which equated to 09E1. So when I put 09E1 and ran the PCM relearn, all of the DTC's cleared. I drove it around and the speed appears to by spot on when compared to GPS, so that's what I left programmed in.

[Salva]

I have change tire size on my Fiesta, that's work perfectly, but i have dtc P106A on PCM.
PCM module initialisation/relearn works on Fiesta Ecoboost 2018 mk8 ? Or i will brick my PCM module or even my car ?

[lenko]

It won't brick your module or your car. It's there to do after you make changes. I've done it with a tire size change to correct speedo a number of times until I got it right.

[Salva]

It won't brick your module or your car. It's there to do after you make changes. I've done it with a tire size change to correct speedo a number of times until I got it right.

Thank you for your answer. Nothing to do after this ? I read for learning pcm we need to drive for 10 miles at various speed ? That's it to scared me.

[lenko]

That's what the relearn is for. You can check for DTC and clear those and also check and see if your speedometer is correct now and make adjustments as needed.

[hiteq55]

Hi all! First time user...long time lurker
Hey any idea which register in the BCM would have the tire circumfernce HEX code for a 2019 Transit T250 Cargo Van?

726-12-01 is only a 2 block register.

Would b great to know what all these registers control

[lenko]

In BCM run Module Configuration - NOT the Asbuilt. A screen will show up listing various categories that you can click on and change.
Scroll down and see if there is a tire circumference line with a number in mm - (I have 2174). Click on the line and change the number to whatever your tire circumference is and then hit write and follow prompts. Then you can run a relearn in PCM and also remove any DTCs that show up.