Setting arbitrary tire sizes for speedometer calibration

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Dordogne
Posts: 13
Joined: Fri Aug 12, 2016 6:21 pm
Vehicle: Mustang 5.0 2017

Setting arbitrary tire sizes for speedometer calibration

Post by Dordogne » Thu Oct 20, 2016 7:12 pm

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.

j.osbourn80
Posts: 7
Joined: Sat Oct 29, 2016 5:01 pm
Vehicle: f250, 7.3 turbo diesel, 2001

Re: Setting arbitrary tire sizes for speedometer calibration

Post by j.osbourn80 » Mon Oct 31, 2016 8:33 pm

I'm trying to recalibrate the speedo on my 2001 f250 superduty for the 35x12.50r17 tires that are on it. PROVING to be difficult to do as I am not certain where to find it or how to do it wi th forscan. Any help would be appreciated.

Dordogne
Posts: 13
Joined: Fri Aug 12, 2016 6:21 pm
Vehicle: Mustang 5.0 2017

Re: Setting arbitrary tire sizes for speedometer calibration

Post by Dordogne » Tue Nov 01, 2016 9:08 pm

j.osbourn80 wrote:I'm trying to recalibrate the speedo on my 2001 f250 superduty for the 35x12.50r17 tires that are on it. PROVING to be difficult to do as I am not certain where to find it or how to do it wi th forscan. Any help would be appreciated.
F150's prior to 2011 definitely won't be able to use this method.

j.osbourn80
Posts: 7
Joined: Sat Oct 29, 2016 5:01 pm
Vehicle: f250, 7.3 turbo diesel, 2001

Re: Setting arbitrary tire sizes for speedometer calibration

Post by j.osbourn80 » Thu Nov 03, 2016 12:57 pm

Would you happen to know the location code in the module for 2001 year models?

Dordogne
Posts: 13
Joined: Fri Aug 12, 2016 6:21 pm
Vehicle: Mustang 5.0 2017

Re: Setting arbitrary tire sizes for speedometer calibration

Post by Dordogne » Thu Nov 03, 2016 4:28 pm

Earlier vehicles have very little or no as-built data at all. I'm not sure, I don't own one to experiment with.

j.osbourn80
Posts: 7
Joined: Sat Oct 29, 2016 5:01 pm
Vehicle: f250, 7.3 turbo diesel, 2001

Re: Setting arbitrary tire sizes for speedometer calibration

Post by j.osbourn80 » Thu Nov 03, 2016 7:57 pm

I've located it I'm the abs module and there is a list of different sizes of tires in there and one choice named other. However my size isn't in there nor can I figure out how to manually enter the size and revs for my tires.

Dordogne
Posts: 13
Joined: Fri Aug 12, 2016 6:21 pm
Vehicle: Mustang 5.0 2017

Re: Setting arbitrary tire sizes for speedometer calibration

Post by Dordogne » Fri Nov 04, 2016 12:42 am

Sounds like you're editing the canned FORScan settings rather than the as-built hex directly.

Also, I suspect whatever data is in the ABS module is separate. The truck uses a sensor in the transmission to drive the speedometer and odometer, not the ABS wheel speed sensors.

j.osbourn80
Posts: 7
Joined: Sat Oct 29, 2016 5:01 pm
Vehicle: f250, 7.3 turbo diesel, 2001

Re: Setting arbitrary tire sizes for speedometer calibration

Post by j.osbourn80 » Fri Nov 04, 2016 2:12 pm

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?

Dordogne
Posts: 13
Joined: Fri Aug 12, 2016 6:21 pm
Vehicle: Mustang 5.0 2017

Re: Setting arbitrary tire sizes for speedometer calibration

Post by Dordogne » Fri Nov 04, 2016 3:55 pm

No, but if FORScan is changing it with the canned values, you need to take a look at your as-built in hex before and after and see if you can work out what's changing and what it means so you can work out how to program an arbitrary size.

Check out my OP for an example of how the hex translates to tire size on newer vehicles

j.osbourn80
Posts: 7
Joined: Sat Oct 29, 2016 5:01 pm
Vehicle: f250, 7.3 turbo diesel, 2001

Re: Setting arbitrary tire sizes for speedometer calibration

Post by j.osbourn80 » Fri Nov 04, 2016 4:00 pm

I understand how that works and used your formulas to calculate my own revs and circumference. But if I go in to edit ASBUILT I have nothing listed, only a series of three boxes at the top that appear to be to search or call up specific memory block locations

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