Setting arbitrary tire sizes for speedometer calibration

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mrpositraction
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by mrpositraction »

I guess I should clarify the DTC that is coming up on the Forscan. It is saying that the data from the BCM isn't recognized which I assume means I've entered a value that is outside the range that is allowed? DTC U0422

Anyone running a 2019 Expedition or even an expedition with the 10 speed? It is about 4-5mph off and I'd just like to get closer, while I would like the speedo to be perfect I am just looking for closer at this point.
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mrmrva
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Vehicle: Fiesta, 1.0 EcoBoost, 74kW, 2014

Re: Setting arbitrary tire sizes for speedometer calibration

Post by mrmrva »

Trying to recalibrate speedo on Fiesta 2014 1.0 EcoBoost.
I write new tire size and circumference in BCM backup then Write to main but cannot see any changes in BCM As built. IPC is showing new data but speedo is still off.

My BCM values in fields from Calculator from first post don't seem valid for tire circumference:

726-12-01 0000 0000 0040

726-15-01 FFFF FFFF FF3E
726-15-02 FFFF FFFF FF3F

Any advices?
lenko
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by lenko »

You had better check out BCM 726-45-01 here: https://docs.google.com/spreadsheets/d/ ... 1329872825

726-12-01 is for newer vehicles staring in 2015 like my 2015 Lincoln MKC. The middle or second set of figures is where you enter your values. There is a place to enter tire circumference in the newer Forscans and once you change that to your calculated circumference in cm, that will make the changes to your 726-45-01 or 726-12-01 - depending on the age of your vehicle. It's a lot simpler just to enter the tire circumference and hit write. Look in either the BCM or IPC (not asbuilt) it's there somewhere. After that you can do the relearn.
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mrmrva
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Vehicle: Fiesta, 1.0 EcoBoost, 74kW, 2014

Re: Setting arbitrary tire sizes for speedometer calibration

Post by mrmrva »

On Fiesta BCM modules go up to 726-16-03
I've changed Tire size and circumference in BCM backup (that one has dropdown menus), wrote it and then wrote to main (Write to main or some command like that). I can see new values in IPC but no value has changed in BCM As Built (I've compared all values before and after)

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My dial speedo is about 10% off which is a lot, but digital one available in Test menu (pressing trip reset before ignition on) is spot on.
lenko
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by lenko »

Calculate your 4 figures of the tire circumference in cm in this calculator conversion and then look for a 4 figure that is the same or similar in the middle 4 numbers. That may lead you to the line for speedo correction where the hex numbers could be entered. In my MKC the decimal 2174 cm calculates to hex 087E and that comes up as 726-12-01 xxxx 087E xxxx. Originally I was 0862. This corrected my speedo.
https://www.binaryhexconverter.com/deci ... -converter
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mrmrva
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by mrmrva »

Thanks for suggestion. I've set tire size to 195/55 R15 which equates to 1869mm circumference.
Only value close to that in BCM As Built is at 726-13-07, when converted to decimal it is 1870. But nothing close to circumference which was originally 1760mm (6E0 in hex) which I've changed to 1820mm (71C hex).

Strange thing I've noticed today is that speedo error is practically the same at 60km/h as at 120km/h, around 5-6 km/h. So it behaves like there is factory offset set not like tire size issue which should give same % at various speeds.

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lenko
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by lenko »

The one that look like a tire setting is 726-10-03 xxxx 07E0 xxxx. The 71C you are getting should be 071C - you need the 0 at the beginning. 07E0 calculates to 2016 decimal. 071C calculates to 1820. There is a relearn in Forscan for various modules, try that. The difference from 1870 to 1820 seems to be valid range for speedo correction. I had to go from 2146 cm decimal to 2174 cm decimal to correct mine. My speedo was showing higher than my actual speed. Are you going faster than your speedo shows? I had to increase my 2146 as I was going slower than speedo.
Last edited by lenko on Mon Mar 08, 2021 7:25 pm, edited 1 time in total.
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mrmrva
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by mrmrva »

I've omitted leading zeros, sorry about that. Since 07E0 (2016 decimal) is larger than 1820, speedo should be showing higher speed than actual, not lower. And size difference is too big, Fiesta cannot take 2016mm tire, so I doubt it's that one.

EDIT: Any idea what Instrument cluster calibration is?

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lenko
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by lenko »

You are correct, I made error with the 1870 figure - don't know where I got that one.
lenko
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Re: Setting arbitrary tire sizes for speedometer calibration

Post by lenko »

I don't see anything in my ICM that would correlate with tire setting. There may be but I just don't know. There might be some setting for wheel ticks which measure speed as well I would research the ABS and transmission modules to see if you can find anything that way. In my 06 Mustang GT it's a lot easier as with my SCT tuner I just set the rear end axle ratio and revs per mile and that does everything. In the 2015 Lincoln MKC it was tougher and took a much longer time to determine.
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