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Old 11-11-2013
RazorEdge's Avatar
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Location: Mount Pleasant
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Tire Sizes...

Can some one explain to this good ol' boy just how the numbers that are listed as tire sizes figure into the actual size of the tire (in inches) and why they are numbered the way they are? I don't know a 225/70R-15 from a doughnut. I would like to understand how the numbers relate to each other and why. I am running 31x10.50x15's right now. But I have no idea how those numbers break down to their equivenent metric size but I'd sure like to know. What number equates to tread width. What numbers equate to tire height? And how do they relate to each other. I have the idea that the numbers might designate a certain ratio of tire width to height and is expressed in millimeters. I want to know just what numbers will give me the tallest and widest tire that will fit on a 15"x10" or a 15" by 8" wheel. What numbers translate to a tire 12.50" or 14.50" wide...what numbers translate into a 33" or 35" tall tire...or for that matter the height of the wheel on a skateboard. But don't just give me the numbers. I want to know the methodology on just how they are figured.

I don't want to go into a tire store and show my ignorance by saying," Hey Dude...I want you to put the tallest and widest tire that you can cram under the fenderwell...and, oh yeah...I want a tread pattern that has the least agressive of those you have...". I want to go into a tire store and be at least somewhat knowledgeable and conversant when the sales man is speaking.

Can someone explain this "mystery" to me?
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Old 11-11-2013
logan03CO's Avatar

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Common Resources:

Metric / Inch Tire Size Converter:Tire Size Converter | Metric to Inch Tire Size Calculator

In-depth Sidewall Markings Definitions & Meanings: http://www.tirerack.com/tires/tirete....jsp?techid=46

Quick Summary ('Borrowed' from the Internet):

We'll first label the parts as follows:


A- This is the width of the tire in millimeters. The conversion is 1"= 25.4mm so for our example: 285/25.4= 11.22" width.

B- This part is the "aspect ratio" of the tire and is a relation of the tire's width to the sidewall height of the tire. In our example it's '70'. This means the sidewall height is 70% (.70 in calculation) of the width of the tire. That means the sidewall height is:

11.22 x .70= 7.85"
BUT there is a 'sidewall' both above and below your wheel when measuring height, so you must multiply this by 2 when calculating the tire's overall height.

C- This third piece is simply your rim size. So, for a final calculation of a tire's size, you add this to the total from part B.

In conclusion, a 285/70/R17 equates to 11.22" wide and is (7.85x2)+17"= 32.71" tall, or basically equivalent to a 33x11.50R17 if we called it a standard size.

How about one more example, 265/75/R16:

Width: 265/25.4= 10.43"
Sidewall: 10.43 x .75= 7.82"
overall height: (7.82" x 2)+ 16"= 31.64" tall. Basically, a 32x10.50R16
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Old 11-11-2013
RazorEdge's Avatar
Thread Starter
Join Date: Nov 2013
Location: Mount Pleasant
Posts: 130
Tire Sizing..

Geeze, Logan...

Every time I post a question it's you that has the answer.

Your explination on how tire sizes are figured (based on the metric/aspect ratio system) was clear and very easy to figure out.

Makes me feel kinda stupid. But I'm learning and hopefully, any questions I have in the future will be fielded by you.

Thanks again

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