### Setting arbitrary tire sizes for speedometer calibration

Posted:

**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 ((

As you would imagine, armed with these formulas you can program

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.

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.