Tire Circumference data collection

[NewBroncoOwner]

Post your tire circumference and tire size to help others when changing tire sizes. I have a base model and changing the circumference size in FORSCAN has been a guessing game since the math formula has produced a number that pushes the speedo is off by negative 3-5 mph. If we can get a thread going to share the circumference numbers that work, or close to the correct speed, that would be very helpful for future users.

Current Tire is a 285/70 R17
circumference for FORSCAN; TBD

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[Fly]

This should help make it easier:

https://tiresize.com/calculator/

 

[Deleted member 32220]

Post your tire circumference and tire size to help others when changing tire sizes. I have a base model and changing the circumference size in FORSCAN has been a guessing game since the math formula has produced a number that pushes the speedo is off by negative 3-5 mph. If we can get a thread going to share the circumference numbers that work, or close to the correct speed, that would be very helpful for future users.

Current Tire is a 285/70 R17
circumference for FORSCAN; TBD

2525 is stock badlands 285/70/17 don’t go by the calculator.

 

[M4Madness]

2525 is stock badlands 285/70/17 don’t go by the calculator.

Someone on here experimented and said 2508 gave them a speedometer reading that perfectly matched their GPS. Reports are that Bronco speedometers read slightly fast, so the difference between the factory 2525 and 2508 may account for that.

 

[NewBroncoOwner]

This should help make it easier:

https://tiresize.com/calculator/

Ive used that and the numbers are off, terribly

 

[NewBroncoOwner]

Someone on here experimented and said 2508 gave them a speedometer reading that perfectly matched their GPS. Reports are that Bronco speedometers read slightly fast, so the difference between the factory 2525 and 2508 may account for that.

Ill try them tomorrow

 

[66Fan]

I'd think the most accurate way would be to measure.

 

[M4Madness]

Ill try them tomorrow

Please do, as I will be putting Badlands take-offs on my Base as soon as it arrives. (It's being built in 4 days.)

 

[NewBroncoOwner]

I'd think the most accurate way would be to measure.

I have with a tape measure and an auto level (im a surveyor) to verify within the tenth, did the math and it still came up with a wrong circumference for the tire size lol

 

[‘21OBX]

Ive used that and the numbers are off, terribly

Part of the issue is to be close to correct you have to measure front the center of the wheel to the ground with the vehicle on the ground and inflated properly. Then do the math from there. I’ll have to get back in forscan to see what my numbers are for my 37’s. I do know the Bronco would not let me enter 2800 or above.

 

[66Fan]

I have with a tape measure and an auto level (im a surveyor) to verify within the tenth, did the math and it still came up with a wrong circumference for the tire size lol

Classic! Sounds like they have converted mm to in and back a couple times to introduce some wicked rounding errors! We used to run into this all the time with some German manufacturers and alignment specifications going degrees/rads to mm to in... just spec it the way the engineers do and let the math happen once!

 

[Hopeless Diamond]

I'd think the most accurate way would be to measure.

The problem is the tire is not perfectly round when it's carrying the weight of a vehicle so circumference/radius/height is off. Some of the tire manufacturers have a listing for revs per mile. When I had my SVT Cobra, the tuner I had for it was looking for that number in order to adjust the speedo. Really wish that was still the typical practice.

 

[66Fan]

The problem is the tire is not perfectly round when it's carrying the weight of a vehicle so circumference/radius/height is off. Some of the tire manufacturers have a listing for revs per mile. When I had my SVT Cobra, the tuner I had for it was looking for that number in order to adjust the speedo. Really wish that was still the typical practice.

Yeah, we used to chalk the tires/floor, roll forward one rev and re-chalk and measure between lines... that accounts for loaded circumference.
The only problem is, we don't know if they spec'd it loaded or unloaded. Automotive specifications leave waaaayyy too much room for interpretation, unless explicitly noted.

 

[BudgetBronco]

I think the best way to do this is:
- Drive on the highway at three different speeds like 55, 65, and 75 and use a GPS to see what actual speed is (for purposes of example here, lets say it is 61, 71, and 82)
- Calculate the ratio of those three readings (in this case: 1.109, 1.0923, and 1.0933)
- Take the average of those three ratios (in this case: 1.0982)
- Whatever value is in the as-built FORscan, multiply it by the average of the ratios (1.0982)

That should give you a really close result

 

[NewBroncoOwner]

Classic! Sounds like they have converted mm to in and back a couple times to introduce some wicked rounding errors! We used to run into this all the time with some German manufacturers and alignment specifications going degrees/rads to mm to in... just spec it the way the engineers do and let the math happen once!

we all know that common sense and a "one standard measurement" are far from being correlated lol

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