How to calculate tire size

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HenryJ
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How to calculate tire size

Post by HenryJ »

It is pretty easy to figure out what size tire a 31x10.5-15 has for diameter, but the metric measurements can confuse some people.

The first number is the width using the metric measurements (195, 205, 235, etc.) usually in millimeters.

The next number is the percentage of width that the sidewall equals. (50=50% , 70=70% , 75=75% , etc.)

The last number is the wheel diameter (14, 15, 16, etc.) usually in inches.

You may find a letter "R" for "radial" , "LT" for "Light Truck" or "P" for passenger, but we are just concentrating on size right now ;)

So to figure out the diameter and width of a metric tire:

We will use a 235/75-15 as the example.

First the width-
That's easy :roll: just convert millimeters to inches :D I find it easier to estimate centimeters than guessing millimeters and have an old Garfield ruler with centimeters on one side and inches on the other. This is an easy way for me to visualize the conversion.
So 235mm is 23.5 cm and 235mm=9.25"

Next the sidewall-
I use a calculator for this , and some easy ones you can do in your head ;)
75% of 9.25" is 6.9"

Now we can figure out the diameter-
Again really easy, top sidewall, wheel and bottom sidewall for the total diameter.
Sidewall x 2 + wheel or , 6.9" x 2 = 13.8" + 15" = 28.8"

Of course there are calculators on the website- Axle Ratio/Tire Size calculator , but being able to visualize what it means and do it in your head is a valuable skill ;)

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