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Actual tire diameter - a scam?

PyrPatriot

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There are 63360 inches in a mile. The circumference of a circle/tire is Diameter multiplied by Pie (3.14159265...). Revolutions per mile is thus 63360/(PIE x Diameter). This becomes 609 = 63360/(3.14159265 x Diameter). Solving for the Diameter we get Diameter = 63360/(3.14159265 x 609), which yields 33.117 inches. Thus your 35” tire is actually 33”. Even accounting for a manufacture’s website’s listing that the overall diameter is something like 34.8 inches, you are leaving customers 1.7” short of tire diameter.

For example: Falken’s 255/75/17 calculate to be 31.2”, but they report 32.3”. Similarly Falkens 35/12.5/17 calculate to be 33.12” while reporting to be 34.8” (close to 35 ok). To get to 35” you have to go to 37” tires
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adbeck

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Where are you getting 609 from?
 

hjdca

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There are 63360 inches in a mile. The circumference of a circle/tire is Diameter multiplied by Pie (3.14159265...). Revolutions per mile is thus 63360/(PIE x Diameter). This becomes 609 = 63360/(3.14159265 x Diameter). Solving for the Diameter we get Diameter = 63360/(3.14159265 x 609), which yields 33.117 inches. Thus your 35” tire is actually 33”. Even accounting for a manufacture’s website’s listing that the overall diameter is something like 34.8 inches, you are leaving customers 1.7” short of tire diameter.

For example: Falken’s 255/75/17 calculate to be 31.2”, but they report 32.3”. Similarly Falkens 35/12.5/17 calculate to be 33.12” while reporting to be 34.8” (close to 35 ok). To get to 35” you have to go to 37” tires
I noticed that tires always measure "short" when measuring with a tape measure and level...., however, when testing the speedo and GPS, it is not off... why ? I believe that as your speed increases, the tire gets a little taller from the centrifugal force and expands to it's correct specified dimension, and the speedo registers correctly at speed.
 

DreamedofaJeepSomeday

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For example: Falken’s 255/75/17 calculate to be 31.2”, but they report 32.3”. Similarly Falkens 35/12.5/17 calculate to be 33.12” while reporting to be 34.8” (close to 35 ok). To get to 35” you have to go to 37” tires
I do see some variation in the calculations from tire to tire. But how do we know the diameter is being reported incorrectly? It could just as easily be the revolutions/mile that is wrong on their charts.
 

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Renegade

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1. Tire “sizes” often aren’t actual
2 They “squish” under weight, reducing the tire’s effective radius
 
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PyrPatriot

PyrPatriot

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Taking a tape measure to my 255/75/17 (aka 33s) I get 31”
:(
 

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PyrPatriot

PyrPatriot

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Falken clarifies

Regarding the Revolutions per mile inquiry, this published value on our website is based on the “SLR” static rolling radius and thus circumference of the tire taking into account load and inflation. This rev/mile value will not make sense if you try to back-calculate the overall diameter, which is measured inflated but not loaded. Here’s a link I found that might help explain:

https://www.sciencedirect.com/topics/engineering/effective-rolling-radius
 

LostWoods

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Too many variables that impact tire diameter. An unmounted tire on its side will be larger in diameter than on end. Once you get it on the wheel, there's a bell curve because too wide or too narrow and you start pulling the edges of the tread which will also impact the diameter of the tire. Then, as was said, tire pressure and vehicle weight impact mounted diameter more than probably anything so there's just too much shit going on and they have to pick a median.

When i worked for Goodyear, the diameter was provided as a static loose tire mounted on a wheel in the middle of the acceptable range and unloaded. Revolutions per mile, however, were given assuming mounted and loaded which will naturally cut from that diameter. Any deviation from the assumed weight and pressure would give a different spec.
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