How to understand metric tire sizes
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: 285/70/R17 _^__^__^ _A__B___C 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! NOTE: 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.:cool: 
Nice post :thumbsup:

Excellent post!! Moved to the Wheels & Tires section and stickied!

good job xtreme!!

Cool, good job Bob. Just seems like there are a lot of people that don't really know a whole lot about this so thought I'd try and help everyone out a little.

very nice, i should print it out and bring it to work..I always get asked about that stuff.
Why cant everyone just make it simple, and make everything in sae sizes so theres no guesswork 
It should be noted that the size on the side of the tire may be the same from one manufacture to another, but the tires may still be slightly different in size. A 285/70/17 from Nitto will be a different than the same size tire from BFG. Only by a few tenths of an inch, but the size does vary.

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:rolleyes:silly US 
Good info! I was going to post something like this today, but you beat me to it. :) I have been racking my brain trying to figure out what size tires I want to get and have been crunching number the last few days. :confused:
I would like to add, if it's not obvious.....Two of the same size tires will take on different overall measurements (width and diameter) depending on what width the rim is. It may not be much, but it happens. 
Now something to think about.........how do you figure in the tread?
We had a 2000 Silverado that had 235/75/16's Goodyear Wrangler factory tires and I put on 235/75/16 BFG mud terrains. The extra tread caused a rub so I had to put a 1.5" lift on the front. It evened out the truck and got rid of the rub. The tire "sizes" were the same but there was almost 3" difference in height. 
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What about the 'R'16 part... It seems to me that I find a lot of people mistaking the 'R' for meaning rim size when it actually stands for the type of tire, in most cases a Radial. Although it can also be VR, HR, and ZR to denote the speed rating.
Not very important for the majority of us, but could be useful information to a few of the x runner guys.:) Or just for the glory of expanded knowledge. 
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Okay, I don't want to kill this subject to death but I purchased a pair of Goodyear Wrangler AP and a pair of Mastercraft Courser AWT, both 225/75/15's, however, I think I made a mistake. Will the different tires mess up my tranny or anything in my truck? I'll eat the lost before I let anything happen to my truck....

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Informative...

Thats what its here for :)

ahhh....I knew this! 8)

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