I was recently PMed by a member for information regarding fitment of tires on his truck, but it was obvious to me that they had very little knowledge as to what the metric sizing numbers really meant. I decided that far too many don't know this valuable information and set out to do a write up thats easy to understand as to what the numbers mean. I also teach how to calculate the sizes to standard US measurements for times when you don't have a size calculator available, like the one on our site. So, anyone that doesn't understand what the numbers mean, Read up so you can understand too!
To help you understand tire sizes a little better here's a quick lesson.
For an example we'll use a fairly common tire size for trucks with a 3" lift, a 285/70/R17.
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
This method of finding a tire size will work for every metric tire size out there!
It should also be mentioned that different manufacturers' tire sizes will slightly vary from these calculations. Its usually a very minor amount but it's still there none the less. For this reason I recommend everyone looking at tires to go to the respective Manufacturers web site and look up the exact specifications they list for the tire before buying a set, or to compare tires between different manufacturers.
Hope this helps someone and that you can calculate tire sizes without having to be reliant on a program.