Tire aspect ratio

[Rexsname]

I understand that on a 255/85-16 that the sidewall is (nominally) 85% as tall as the tread is wide. On a 33x10.50-15 what would the aspect ratio be? I have no real need to know this other than I'm kinda anal about knowing usless stuff like this. What does the math problem look like?


REX

 

[jcbrandon]

That feels like some sort of Jedi mind trick this late on a Sunday night.

I can't write the formula. But I can tell you that tirerack.com has great specs on their website.

Here are the numbers on those two sizes of a BFG Mud-Terrain T/A KM:

255/85/16"
max load:3000 lbs.
section width: 10"
overall diameter: 33.3"

33x10.5 15"
max load 2600 lbs.
section width: 10.5"
overall diameter: 32.8"

If I remember algebra correctly (not likely) all the information is in there to solve the problem.

 

[HenryJ]

33 inches tall, subtract the wheel diameter for 18 inches, divide by two for 9 inches sidewall. 9 inches is pretty close to 85% of the tread width.

255 is 10 inches wide having a 8.5" sidewall multiplied by two , plus the wheel diameter gives us 33 inches tall. That makes it a 33x10-16

 

[Rexsname]

My mind is working more clearly this morning..........9" divided by 10.5" gives us .857

REX

 

[Azlugz]

this would be closest to a 265/85R-15 as partially stated.

1" = 25.4 mm so you would take the width of 10.5 times the 25.4 to get the tread.

As stated the sidewall is the diameter minus the rim all divided by 2 and then that divided by the section width of 10.5...this giving .857 or the common 85%

To work it backwards for a metric tire to find the approimate SAE size -
in the 255/85R-15

Section width in mm / conversion of inch to mm = Section width in Inches
255 / 25.4 = 10.04 so a 10.0 section width

Section width in inches * aspect ratio = Sidewall height
10 * .85(85%) = 8.5

sidewall height * 2 + rim diameter = Total height
8.5 * 2 + 15 = 32

This tire is a 32 x 10.0 x 15

Just spelling it out a little more for anyone else reading that was not sure what was being reffered too.

 

[Pskhaat]

http://xepoch.com/mud/tire.cgi

 

[blatant]

try this


http://www.miata.net/garage/tirecalc.html

 

[HenryJ]

Here is another one from my website: Axle Ratio/Tire Size Calculator

 

[madizell]

I agree with the math stated above, but would note that section width is not used in determining aspect ratio. Instead, you use tread width.

Didn't find a 33x10.5x15 listed, but found a commonly used tire, Cooper Discovery S/T in 33x12.5x15LT to use as an example:

This tire has a diameter of 32.63 inches, a section width of 12.36 inches, and a tread width of 9.8 inches.

The sidewall height would be computed as: 32.63 - 15 divided by 2 = 8.815

Aspect ratio is the ratio or fraction expressed as sidewall height over tread width: 8.815/9.8 = .899. Rounded, these would be 90 aspect ratio tires.

 

[w_b]

I agree with the math stated above, but would note that section width is not used in determining aspect ratio. Instead, you use tread width...

Okay, the thread is a bit stale, but I've been dredging up old threads looking for tire info. I couldn't wrap my mind around this after I started doing the math for some metric/ratio conversions. Tire Rack says we use the section width, not the tread width to determine aspect ratio. Or maybe I'm missing your point.

From TireRack:
"Section Width

Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.

P225/50R16 91S

The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch)."

Dunlop's site gets more to the point:
"ASPECT RATIO: This two-digit number indicates the tire's aspect ratio. It compares the tire's inflated section height, which is the distance from the bead to the tread, to its section width (maximum)."

Bridgestone gives their tread widths on their website if it'll help.