Tire Size Calculations
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Joined: Mar 2007
Posts: 124
From: New Mexico
Tire Size Calculations
Hello all, I haven't spotted anything like this on here yet, so I figured I would post some helpful info.
A standard metric tire size, as most know, is shown like this: 185/60/15. The first number is your section width (Tire Width), and the next number is the sidewall height using the aspect ratio of the section width (%), and the last number is your rim size. The way you break this down is as follows.
Approximately 25.4mm=1"
So, take the first number which is 185, and divide it by 25.4 which equals 7.28" This is the width of the tire.
Next, you take the aspect ratio, which in this case is 60%; so take 7.28X.60 which equals 4.37" This is your sideall height
Finally, you will take your Rim size, plus your side your sidewall heightx2 (One for each side of the rim) 15+4.37X2=23.74"
Therefore, with a 185/60/15 tire, the overall height is 23.74", and it is 7.28" wide.
These calculations can be used when you are trying to figure up how a different wheel and tire set up will compare to your current one.
This will work with any metric tire size, regardless of rim size. All that is changed is you add the different rim size in with the rest of the tire info to get your overall diameter.
Now obviously, not everyone wants to break all of this down, so here is a handy little site that can be used. Tire Size Calculator - Discount Tire Direct
This is a very helpful site due to the fact you dont have to calculate everything out, and it also shows how far off your speedometer will be.
The only thing that the site, nor the calculations will help someone out with, is say running a tire that is too narrow for a rim (stretch). When you stretch the section width of a tire, the sidewall height will shrink, due to the fact that it is tapered in towards the center of the rim.
Hopefully this will help some folk out that are confused when they are looking at tire sizes!
Dustin
A standard metric tire size, as most know, is shown like this: 185/60/15. The first number is your section width (Tire Width), and the next number is the sidewall height using the aspect ratio of the section width (%), and the last number is your rim size. The way you break this down is as follows.
Approximately 25.4mm=1"
So, take the first number which is 185, and divide it by 25.4 which equals 7.28" This is the width of the tire.
Next, you take the aspect ratio, which in this case is 60%; so take 7.28X.60 which equals 4.37" This is your sideall height
Finally, you will take your Rim size, plus your side your sidewall heightx2 (One for each side of the rim) 15+4.37X2=23.74"
Therefore, with a 185/60/15 tire, the overall height is 23.74", and it is 7.28" wide.
These calculations can be used when you are trying to figure up how a different wheel and tire set up will compare to your current one.
This will work with any metric tire size, regardless of rim size. All that is changed is you add the different rim size in with the rest of the tire info to get your overall diameter.
Now obviously, not everyone wants to break all of this down, so here is a handy little site that can be used. Tire Size Calculator - Discount Tire Direct
This is a very helpful site due to the fact you dont have to calculate everything out, and it also shows how far off your speedometer will be.
The only thing that the site, nor the calculations will help someone out with, is say running a tire that is too narrow for a rim (stretch). When you stretch the section width of a tire, the sidewall height will shrink, due to the fact that it is tapered in towards the center of the rim.
Hopefully this will help some folk out that are confused when they are looking at tire sizes!
Dustin
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Joined: May 2007
Posts: 4,997
From: Calgary, Alberta
Nice post!
One thing to note, this is a theoretical diameter ONLY!
Manufactures have pretty ridiculous tolerance allowances given to them by the DOT and TC here in Canada.
As an example, here in Canada I was shocked to learn that an allowance of 7% was tolerable. I had a tire that measured 218mm across the face on a 205/40 tire. I was going to complain about the manufacturer until I learned that legally they could sell me a 215/40 labeled as a 205 just because of the allowable tolerance. In case anybody is wondering the brand is LEXANI
Long story short.... buyer beware and it never hurts to do some digging about which brands tend to be larger or smaller than the theoretical diameter!
One thing to note, this is a theoretical diameter ONLY!
Manufactures have pretty ridiculous tolerance allowances given to them by the DOT and TC here in Canada.
As an example, here in Canada I was shocked to learn that an allowance of 7% was tolerable. I had a tire that measured 218mm across the face on a 205/40 tire. I was going to complain about the manufacturer until I learned that legally they could sell me a 215/40 labeled as a 205 just because of the allowable tolerance. In case anybody is wondering the brand is LEXANI
Long story short.... buyer beware and it never hurts to do some digging about which brands tend to be larger or smaller than the theoretical diameter!
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