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Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Fri Dec 09, 2016 11:32 pm
by JPMcNown
20161209_173839-1029x855.jpg
20161209_173839-1029x855.jpg (239.05 KiB) Viewed 24791 times
I have a 2013 Ford Fusion SE Energi, and I'm not confidant 726-18-01 is where I wanna be making changes to my tire size. Is there a way to know for sure? OEM tire size is a 225-50-17, and I changed to a 225-45-17. Its enough of a difference in my speedometer that I'd like to correct it.

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Fri Mar 17, 2017 5:23 pm
by 1995E
Dordogne wrote:I've worked out how to program any tire size you want in the BCM, which enables you go get a very accurate speedometer reading even if you have oversized non-factory tires.

In all of the current model year cars I've looked at (Explorer, Fusion, Mustang, F150) the tire size is stored in the second four-digit hex block of 726-12-01 as follows:

726-12-01 xxxx 0840 xx

If you convert the hex to decimal, what you get is the circumference of the rear tire in millimeters - but with some sort of revs/mile conversion factor applied. What this conversion factor is can vary depending on year. I did most of my initial research on 2016-2017 Mustangs, and on those cars it was very close to 96.7% of the nominal tire circumference. This conversion factor also seems to work for newer Explorers and F150s, and 2017+ Fusions. I found that 2016 and earlier Fusions seemed to use a conversion factor of 97.7%.

So for example, let's take the rear tire on my 2017 Mustang GT with Performance Pack. The rear tires are 275/40R19. The formula to figure the nominal circumference of that tire size is ((275 * 40 * 0.02) + (19 * 25.4) ) * Pi which gets you 2207mm rounded off. Multiply that by the conversion factor of 96.7% and you get 2134mm rounded off. Convert 2134 to hex and you get 0856. Compare that to the factory as-built for my car which is 0858 - pretty close, off by a whole 2 millimeters.

As you would imagine, armed with these formulas you can program any tire size, not just ones that came as factory options on your car. And if your speedometer is still off, you can measure it against an accurate source like GPS, and use that to refine your value even further.

I came up with a spreadsheet to do these calculations for you which is here: https://docs.google.com/spreadsheets/d/ ... edit#gid=0

Now, as I mentioned this data is stored at 726-12-01 xxxx 0000 xx for the current model year cars that I've looked at. However, older vehicles store this data in a different location in the BCM and the year it changes isn't consistent for all Ford vehicles.

List of vehicles where this info is stored in 726-12-01:

2015-current Mustang
2015-current F150
2014-current US Fusion
2016-current Explorer

List of vehicles where this info is stored in 726-45-01:
2011-2014 F150
2011-2015 Explorer

List of vehicles where this info is stored in 726-18-01
2013 US Fusion

I'm sure there are a lot more vehicles out there we can figure this out on. Sure beats paying $200+ for some sort of aftermarket calibrator, or a dongle that splices in between the transmission speed sensor and the wiring harness.
You're spot on about the location for the 2011-2015 Ford Explorers. I see that changing the ---- XXXX ---- values changes the 4 digits after it automatically. Also, I tried changing it with your formula and your spreadsheet and inputting the hex values, but it does not change the speedometer. Is there an extra value I need to account for to force the BCM to accept the inputted hex value?

Thank you.

EDIT: It seems to me that no matter what tire size I try with a generated hex, it would not change for my 2011 Explorer model year. I think the hex values just corresponds to the preprogrammed tire sizes. Any invalid hex would just force the BCM to default to the previous tire size setting.

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Mon May 22, 2017 9:02 pm
by akcooper9
j.osbourn80 wrote:You are correct, I am editing the canned FORSCAN settings for the ABS module. It did however change the speedometer and odometer calibration. Is there any documentation that contains year and model specific memory block locations for ASBUILT editing so I will know where to look?
Did you ever get this to work?

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Wed Jun 07, 2017 1:12 pm
by Andrew1918
Dordogne wrote:I've worked out how to program any tire size you want in the BCM, which enables you go get a very accurate speedometer reading even if you have oversized non-factory tires.

In all of the current model year cars I've looked at (Explorer, Fusion, Mustang, F150) the tire size is stored in the second four-digit hex block of 726-12-01 as follows:

726-12-01 xxxx 0840 xx

If you convert the hex to decimal, what you get is the circumference of the rear tire in millimeters - but with some sort of revs/mile conversion factor applied. What this conversion factor is can vary depending on year. I did most of my initial research on 2016-2017 Mustangs, and on those cars it was very close to 96.7% of the nominal tire circumference. This conversion factor also seems to work for newer Explorers and F150s, and 2017+ Fusions. I found that 2016 and earlier Fusions seemed to use a conversion factor of 97.7%.

So for example, let's take the rear tire on my 2017 Mustang GT with Performance Pack. The rear tires are 275/40R19. The formula to figure the nominal circumference of that tire size is ((275 * 40 * 0.02) + (19 * 25.4) ) * Pi which gets you 2207mm rounded off. Multiply that by the conversion factor of 96.7% and you get 2134mm rounded off. Convert 2134 to hex and you get 0856. Compare that to the factory as-built for my car which is 0858 - pretty close, off by a whole 2 millimeters.

As you would imagine, armed with these formulas you can program any tire size, not just ones that came as factory options on your car. And if your speedometer is still off, you can measure it against an accurate source like GPS, and use that to refine your value even further.

I came up with a spreadsheet to do these calculations for you which is here: https://docs.google.com/spreadsheets/d/ ... edit#gid=0

Now, as I mentioned this data is stored at 726-12-01 xxxx 0000 xx for the current model year cars that I've looked at. However, older vehicles store this data in a different location in the BCM and the year it changes isn't consistent for all Ford vehicles.

List of vehicles where this info is stored in 726-12-01:

2015-current Mustang
2015-current F150
2014-current US Fusion
2016-current Explorer

List of vehicles where this info is stored in 726-45-01:
2011-2014 F150
2011-2015 Explorer

List of vehicles where this info is stored in 726-18-01
2013 US Fusion

I'm sure there are a lot more vehicles out there we can figure this out on. Sure beats paying $200+ for some sort of aftermarket calibrator, or a dongle that splices in between the transmission speed sensor and the wiring harness.
hi dordogne
for 285/50r/20 should be 9BB?
is that correct?

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Wed Aug 30, 2017 1:41 am
by dstangland
I did this on my 2017 super duty and it worked great. My speedo matches my gps. Only thing is that I have a check engine light on now.. How can I remove this?

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Wed Aug 30, 2017 2:20 pm
by Dordogne
dstangland wrote:I did this on my 2017 super duty and it worked great. My speedo matches my gps. Only thing is that I have a check engine light on now.. How can I remove this?
The FORScan devs are working on a PCM relearn procedure that's available in beta versions. Check this post out.

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Wed Aug 30, 2017 2:27 pm
by Dordogne
1995E wrote:EDIT: It seems to me that no matter what tire size I try with a generated hex, it would not change for my 2011 Explorer model year. I think the hex values just corresponds to the preprogrammed tire sizes. Any invalid hex would just force the BCM to default to the previous tire size setting.
There's some sort of generational differences in the modules/as-built that determine whether this will work. AFAIK, it only works on the current generation.

For example, it works in 2015+ Mustangs and F-150's, and 2014+ Fusions.

This is not unique to this mod - lots of things changed!

Also, it's not strictly tied to model generations. Although the Mustang and F150 started their current generation in the 2015 model year, the current Fusion generation started in the 2013 model year. In 2014, it got bumped up to the current generation of as-built. So this mod specifically won't work on 2013 Fusions even though they look identical to 2014 Fusions.

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Wed Sep 13, 2017 11:28 pm
by Mustang5L5
2014 Ford Taurus

It's located in 726-45-01 xxxx XXXX xxxx

Canned FORScan values for the OEM tire sizes available. In my case my values are "0873" in HEX which converted to decimal equals 2163.

2163 = 255/45/19
2169 = 235/60/17
2174 = 235/55/18
2213 = 245/45/20

Looks like the conversion factor is also pretty close to 96.7% as well


EDIT: I've not yet tried to change the value to see if it works.

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Tue Sep 19, 2017 7:37 pm
by EMC2
Has anyone successfully completed a tire size change for 2011-2014 F150 using this method?

Re: Setting arbitrary tire sizes for speedometer calibration

Posted: Sat Sep 23, 2017 3:25 pm
by Dordogne
EMC2 wrote:Has anyone successfully completed a tire size change for 2011-2014 F150 using this method?
To my knowledge - no. The 2014 and earlier trucks are on the last generation of as-built as I mentioned, and this doesn't seem to work in any Ford vehicle that isn't on the current generation of as-built.

Side note: is there a way of referring to the different generations of as=built that I'm not aware of?

HOWEVER, the FORScan devs are working on adding a PCM module initialization procedure to relearn vehicle data, and this apparently reads the BCM data for tire size into the PCM on the newer vehicles. This has been used with some success to eliminate the DTC you get from this mod in 2015+ F150's. I do wonder if something like this is what's needed to make the mod work in earlier gen as-built vehicles. See the tail end of this thread: https://www.f150forum.com/f31/correct-y ... 73/index6/