OEM Tire Circumference Values

[BasedBronco]

I've seen a few threads where folks ran into the same problem I did: they put tire/wheel takeoffs from another trim onto their Bronco and their speedometer is a bit off after calculating the nominal tire circumference from measurements. One manual solution to this is linked from flsdiver's F150 FORScan spreadsheet, which is manual GPS truing.

In my case, I put OBX takeoffs on my Base and figured I trust Ford engineers enough to have calculated the best circumference value to use. So, I found a VIN for a Bronco that shipped with these and extracted the value from its as-built data. Here's how.

1. Find a Bronco listing with the OEM tires you're putting on. In my case, Google gave me multiple dealership listings with, "2023 Outer Banks Bronco VIN".
2. Paste that VIN into Motorcraft's As-Built data lookup tool and download the db file, or skip to step 4 to calculate manually.
3. [Optional] Download and import the .db file into my CyberChef recipe* to skip step 4. In CyberChef, click "Open file as input" on the top right to load your file and the output will be the VIN's default circumference value.
   * The recipe link breaks due to overzealous URL sanitization. Paste this into your URL bar:

   https://gchq.github.io/CyberChef/#recipe=XPath_expression('/AS_BUILT_DATA/VEHICLE/BCE_MODULE/DATA%5Bstarts-with(@LABEL,%22726-15-0%22)%5D/CODE/text()','')Drop_bytes(10,2,false)Take_bytes(8,4,false)From_Base(16)

4. You need the first two bytes/characters from the third code value of 726-15-01 and the first two bytes from the first code value of 726-15-02. An example is highlighted in red, below.
   
   
   
   In this example, converting 0A5D into base10 with a base16 converter results in 2653, the circumference value in mm. Input the values you find into the converter instead.
5. Follow the FORScan forums guide for editing your tire circumference value and recalibrating the speedometer.

So as not to hammer Motorcraft with lookup requests, I can try to keep this thread updated with values anyone mentions in case that's helpful to people. I checked the first working VIN that had a tire size included on the sticker/specs, so I encourage you to trust but verify.

Trim(s)	OEM Tires	OEM BdyCM Circumference
Base	255/70R16	2320
Big Bend	255/75R17	2475
Black Diamond	265/70R17	2440
Outer Banks	255/70R18	2475
Badlands	285/70R17	2653 *may be 2610
Wildtrak First Edition Heritage & Limited Sasquatch upgrade	315/70R17	2653
Raptor	315/80R17	2753

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[tractorcrusher]

This is good info! I need to recalibrate my speedo since going to 35s... Else I'm going to get a speeding ticket.

 

[CatMonkey]

I'm not trying to overtake this thread, but there might be a simpler way to go about this. The value needed for this calculation is based on the loaded radius of the tire (with the full weight of the vehicle) and there is always some variation in this data between Manufacturer's for a given tire size. They're not usually a material difference. Almost all manufacturers publish a "revolutions per mile" spec that's should make the value you use for the speedo correction very accurate. Check out the tire manufacturer's website for that spec or one of the big tire wholesalers like TireRack. Essentially the rev/mile spec is how many times a tire will rotate over a given mile (or km for imperial measurements) and to get the circumference data you would divide the number of millimeters in a mile (1,609,344) by the rev/mile spec.

To test this, let's take the data for the Goodyear Wrangler MT in 315/70-17. If we look it up on TireRack, we get the following data. Revs per mile = 608.

If we divide 1,609,344 by 608, we get 2,647. The interpolated data from Forscan above is 2,654, which I've not recalculated, but it is very close to a rev/mile spec of 606. The difference between the two values is not quite 2/10 of a percent or a difference of .14 mph at 70mph. Why the difference, you might ask? if you note the "measured rim width" above, it's an inch wider than the stock wheel width. Perhaps it's tire pressure. That could account for the difference, but that sort of uber accuracy is not necessary. You can usually tell how accurate you are if you use mile marker differences over 100 miles against your odometer or trip odometer reading over that distance.

Can't find rev/mile data fo your tire combination? Try this. Get a measurement of the radius of the tire from the road surface with the full weight of the vehicle on the tire. There will be a certain amount of compression of the radius under a load, so using inflation diameters without a load will throw off the accuracy off 3 to 5%. 5% at 70 mph is about 3.5 mph. Multiply that by 2 to get the "loaded" diameter and then by pi (3.14159). Assuming you used inches, divide 63,360 (inches in a mile) by your "loaded" circumference and there is your revolutions per mile. Overinflated and underinflated tires can also influence the measurement, so do this calculation with your normal inflation pressure for highway use.

 

[CatMonkey]

If you're bad at math or just lazy, you can choose the closed value to your rev per mile data and use the circumference for that value. These ranges are good for tire diameters between 30" to 39".

 

[Bikeric]

If you're bad at math or just lazy, you can choose the closed value to your rev per mile data and use the circumference for that value. These ranges are good for 33" to 39".

View attachment 485011

FORScan does not accept circumference values greater than 2800. Inputting 2801 will brick it and cause multiple "4WD Malfunction" issues. Ask me how I know

 

[CatMonkey]

FORScan does not accept circumference values greater than 2800. Inputting 2801 will brick it and cause multiple "4WD Malfunction" issues. Ask me how I know

Good info. Ford usually puts upper and lower limits for these types of parameters. Another is axle ratio. I just don't know where they are. Obviously you found the upper limit. Those fields are usually adjustable with tuning software once someone breaks the codes to the PCM and finds access to those limits. So have faith, you may have an accurate speedometer one day. The Braptor may have higher limits for larger values.

 

[Baja Bronco]

FORScan does not accept circumference values greater than 2800. Inputting 2801 will brick it and cause multiple "4WD Malfunction" issues. Ask me how I know

Thanks for that information! As I know the feeling! Didn’t know that, now wondering how to change for 37s? Doing the change from inches to millimeters I came up with 2946. I’m going to need to recheck this, or live with the wrong speed.

 

[Broncenstein]

there had to be an error in OP's chart. the 285/70R17 has the same MM as the SAS tires which are 2" bigger. can you double check the MM for the 285/70/r17? im going crazy trying to constantly tinker with the size in forscan lol using the size you listed it's 3mph too slow

 

[Ducati1098]

Thanks for that information! As I know the feeling! Didn’t know that, now wondering how to change for 37s? Doing the change from inches to millimeters I came up with 2946. I’m going to need to recheck this, or live with the wrong speed.

The calculations don’t add up correctly. My speedometer was dead on with every GPS for my 37’s at only 2800mm, even though the calculated number was much higher.

Also, I was able to go up to 2880 before I had any issues with faults that wouldn’t go away.

If you are going to run 37’s, I suggest starting at 2800, checking It against a GPS speed and adjusting from there.

 

[Baja Bronco]

The calculations don’t add up correctly. My speedometer was dead on with every GPS for my 37’s at only 2800mm, even though the calculated number was much higher.

Also, I was able to go up to 2880 before I had any issues with faults that wouldn’t go away.

If you are going to run 37’s, I suggest starting at 2800, checking It against a GPS speed and adjusting from there.

To tell you the truth, this time I didn’t even bother trying. I was able to ad Baja, and rock crawl. I’m 3mph off, and I can live with it for now. Going to Baja in a week, and I’m getting ready. I’ll deal with it when I get back, but thanks.

 

[HoosierDaddy]

I'm not trying to overtake this thread, but there might be a simpler way to go about this.

Simple?
There might very well be, but it sure as hell isn't in your post!!! LOL

DISCLAIMER: I have ZERO real world experience with this, however...


There was a formula posted on an F150 forum I happened a upon a long time ago...

Everybody was using the circumference measurement for their tires (per tire manufacturer) straight up and MOST were reading at least a few mph off.
Some dude replied, well yeah you need to allow for scrub and factors x, y & z, blah blah blah .... just multiply your tires circumference (in mm) by .967...

The next bunch of posts were,"Holy crap, it works!!! I'm spot on with my GPS now!!!"

Sometimes forscan won't accept that exact number, so you need to find the closest one it will take and most end up within 1 mph @70/80 mph.

Just what I read on the interwebtubes, take it for what it's worth.

 

[e90steve]

Does anyone know the factory calibration number for 315/70/17? Thanks in advance!

 

[2022 base]

there had to be an error in OP's chart. the 285/70R17 has the same MM as the SAS tires which are 2" bigger. can you double check the MM for the 285/70/r17? im going crazy trying to constantly tinker with the size in forscan lol using the size you listed it's 3mph too slow

Try 2610, thats the correct mm for 285/70R17

 

[Broncenstein]

Try 2610, thats the correct mm for 285/70R17

To get my speedometer correct using a GPS, I only added like 15mm to the factory Big Bend size. I tried 2610 and it was 3mph fast

 

[2022 base]

To get my speedometer correct using a GPS, I only added like 15mm to the factory Big Bend size. I tried 2610 and it was 3mph fast

hmmm....that is the correct mm for that size tire. not sure then

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