Tire Circumference data collection

[NewBroncoOwner]

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

as for the math and measurements;
285 /70 R17
32.7 inches x 3.14 = 102.68
102.68 inches x 25.4 = 2608.072 mm

With that circumference I get -3mph on the dash compared to GPS at speeds of 25, 35, 45. At 65, 70 and 75 I see a -5 mph on the dash compared to a GPS.

I have lowered it to 2525mm and will post what the numbers I get later today.

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[BudgetBronco]

as for the math and measurements;
285 /70 R17
32.7 inches x 3.14 = 102.68
102.68 inches x 25.4 = 2608.072 mm

With that circumference I get -3mph on the dash compared to GPS at speeds of 25, 35, 45. At 65, 70 and 75 I see a -5 mph on the dash compared to a GPS.

I have lowered it to 2525mm and will post what the numbers I get later today.

What was the as-built number from the factory?

 

[NewBroncoOwner]

What was the as-built number from the factory?

255/70R16 (Stock Tires)
Stock programmed in Bronco Base Computer
29.09 Inches x 3.14 = 91.3426 Inches
91.3426 inches x 25.4 = 2320.10204 mm

 

[NewBroncoOwner]

**UPDATE**

285/70R17 BFG KO2s Load C @38psi

With the tire circumference loaded at 2525mm, reduced from 2608mm, the speed on the dash read 30mph, GPS read 29.97mph. I may jump into FORSCAN and load the 2508mm to see if that -17mm from 2525mm makes a difference.

 

[NewBroncoOwner]

*CONFIRMED*
LT285/70R17 needs a wheel circumference of 2508mm entered into FORSCAN to be 1 for 1 accurate to GPS speed.

Please add your circumference in mm's if running a 35", 37" or even 40" tire. This will help others dial in the speedometer when changing tire sizes.

 

[jenkinslonewolf]

I’m running 315/70 R17 SAS tires. Goodyear Territory. Tire size calculator says they are 2741mm but I entered 2600mm in Forscan and it’s bang on

 

[Chawkbone]

I’m running 315/70 R17 SAS tires. Goodyear Territory. Tire size calculator says they are 2741mm but I entered 2600mm in Forscan and it’s bang on

I found that to be the case as well! Running Toyo Open Country III’s 315/70 R17

 

[kirnma]

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

Yeah, this is exactly what I've been doing. I installed OBX take-offs on my Base and there is roughly a 5% difference in the the speed as measured by my GPS and my Bronco. I've ordered a compatible ODB device and will download FORscan and start by increasing the tire circumference by 5%, and go from there.

 

[JohnnyBronco]

This should help make it easier:

https://tiresize.com/calculator/

Diameter and ROLLING circumference (not to be confused with measuring tore with a tape) can/will/do differ between tire manufacturer for exact same listed size and tire type. A 10 ply tire of same size of a 4 ply will have a larger rolling circumference (maybe not much but measurable) than the 4 due to less sidewall flex.

Better source to compare diameters and rolling circumference for particular tires is www.tirerack.com

 

[RHeinz]

The tire “effective” diameter also changes with the rotational speed. Watch a Top Fuel Drag Car doing a burnout. The tire geometry changes significantly.

 

[Fly]

The tire “effective” diameter also changes with the rotational speed. Watch a Top Fuel Drag Car doing a burnout. The tire geometry changes significantly.

This is a great way to describe it. Thanks for helping me get a better understanding.

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