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How to understand metric tire sizes

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Old 01-13-2009, 04:56 PM   #1
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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.
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Old 01-13-2009, 05:22 PM   #3
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Excellent post!! Moved to the Wheels & Tires section and stickied!
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Old 01-13-2009, 05:42 PM   #4
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good job xtreme!!
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Old 01-13-2009, 05:43 PM   #5
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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.
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Old 01-13-2009, 05:45 PM   #6
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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
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Old 01-13-2009, 05:47 PM   #7
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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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Old 01-13-2009, 05:47 PM   #8
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Quote:
Originally Posted by wildjerseyfirefighter View Post
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
'cuz that'd be too easy Oh and because the US uses SAE and everyone else uses the metric system of measurements.
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Old 01-13-2009, 05:49 PM   #9
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Quote:
Originally Posted by chris4x4 View Post
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.
yeah, that's something that i actually mentioned in the PM i sent back but i didn't put it in the post. I'll change that, thanks for reminding me Chris.
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Old 01-13-2009, 06:00 PM   #10
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Quote:
Originally Posted by xtreme450 View Post
'cuz that'd be too easy Oh and because the US uses SAE and everyone else uses the metric system of measurements.

silly US
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Old 01-13-2009, 06:23 PM   #11
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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.

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.
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Old 01-13-2009, 06:42 PM   #12
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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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Old 01-13-2009, 07:01 PM   #13
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Quote:
Originally Posted by thebigk View Post
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.
I'd think that it could be the difference in tread wear, as well as maybe the profile since many different tires have different shapes to them. Some have a real square tread surface, some round off near the edges, and some tires have big sidewall lugs on them. These can all affect fitment. Simply having the same size tire as before, or say when comparing 285s vs other 285s, sizes will always vary and fitment cant be guaranteed. This is one of the headaches when researching to get a specific tire size, especially if its supposed to be the maximum diameter that fits your vehicle. Another possibility would be accidentally getting a tire in the wrong aspect ratio. ie a 255/75/x is far smaller than a 255/85/x. and i can see this as being an easy to make mistake.
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Old 01-19-2009, 10:21 PM   #14
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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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Old 01-19-2009, 11:44 PM   #15
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Quote:
Originally Posted by SC4333 View Post
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.
Yeah didn't really think about adding that but like you said, it would be more for the performance and speed guys with x-runners and such. There aren't a lot of those guys around here so it probably isn't that necessary. Good to mention though.
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Old 02-06-2009, 08:58 PM   #16
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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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Old 05-27-2009, 10:38 PM   #17
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Quote:
Originally Posted by minuspride View Post
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....
2wd truck? if so, then I dont think there is anything to 'mess up' if 2 tires were slightly different in size. If it is a 4x4, and the front and rears were different diameters then there would be an issue with drivetrain bind in 4x4. I think that if the tires are supposed to be the same size, as you say, that they should still be close enough regardless.
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Old 06-22-2009, 04:29 PM   #18
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Informative...
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Old 08-04-2009, 03:04 AM   #20
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ahhh....I knew this! 8-)
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