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Old 01-29-2013, 01:27 AM   #1
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Default Let's Explain Metric Tire Sizes Shall We? + Misc. Tire/Wheel Specs

Alright fellow F150 lovers, I have always noticed some confusion on the forum when it comes to Metric Tire sizes. Unfortunately this is not an uncommon problem and people who mean the best can unwittingly provide information on a tire size that will cause someone to pick the wrong size, or someone who is sure they’ve finally got this crazy measuring system figured out will purchase the wrong tire for their own trucks. Don't worry though, like I said it is not uncommon at all! I just always tell myself "I'm American. I don't like Metric. Metric is weird and it is way overcomplicated!"



And honestly it is a little overcomplicated. However I will explain and break down here just exactly what all those little numbers on your tires sidewall mean, and then some. Once you understand what those numbers are telling you it will suddenly make a lot of sense and not seem so complicated.



First, the problem:

People seem to get confused with the first 3 digit number on the sidewall. Due to the more recognizable sizing system by Americans, the High Flotation Size (ex. 35x12.50R16), their seems to be the misconception that this first 3 digit number on the Metric Sized tire is in fact the Tires Diameter. This is actually false.



Click the image to open in full size.Source: DiscountTire.com



The first 3 digits you see on this sidewall underlined in Red is called the "Sectional Width."



The Sectional width is the width in millimeters (because we are talking Metric here) and is measured from the outer sidewall to the inner sidewall. This does not include any raised lettering on the sidewall nor any Side Bite Lugs commonly found on Mud Terrains, which would be the Overall Width. This is also not the width of the tread.



Click the image to open in full size.Source: TireRack.com



So, knowing this. Let’s convert some metric sizes into a more recognizable number.



Here is a common stock size 275/55/20 this is a 32" tall tire but it is 275mm wide (or 10.82" wide). In High Flotation this would be read 32x11.00R20 it's not exactly that but that is the closest size in High Flotation.

To find the sectional width in inches simply take that sectional width number of 275mm and divide it by 25.4
275 / 25.4 = 10.82"

Let’s take another size 275/65/20 this is a 34.1” tall tire, yet notice it has the same first 3-digit number as our last tire. In High Flotation this would be read 34x11.00R20 but there is no such High Flotation size that I know of. You’ll understand by the end of this thread why these are a different size yet have the same first 3-digit number.

Lastly let’s take 275/70/18 this is a 33.2” tall tire and again has the same first 3-digit number as the last 2 tires and in High Flotation would be read 33x11.00R18



Click the image to open in full size.Source: DiscountTire.com



The next number underlined in Blue is the Sectional Height or more commonly Aspect Ratio. This number is the most complicated out of them all. The Aspect Ratio is a percentage number and is telling us how tall the sidewall is on one side of the wheel. It's saying that your sidewall height is XX% of your XXXmm Sectional Width. In other words the sidewall heigth is relative to your sectional width.

So if we have a 275/70/18 metric tire, we know that 275mm is 10.82in. Now we take the Aspect Ratio of 70 (which is a percentage) and multiply it by the sectional width. So 275mm x .70 = 192.5mm or 10.82in x .70 =7.574in. This means that our sidewall is 192.5mm or 7.6” tall. Now keep in mind like I said earlier this is only one sidewall on one side of the rim. Remember if you measure from the ground to the top of your tire you are measuring your sidewall height twice. The metric sizing system doesn’t account for this so when attempting to calculate tire diameter you must calculate this number twice or just multiply it by x2.



Click the image to open in full size.Source: DiscountTire.com



Almost Done! The last number underlined in Pink is as everyone knows, the Rim Diameter. This is the easiest of all since it is still in inches.



Now let’s convert a Metric Tire size into High Flotation: (275/70/R18)

So we know our Sectional width is 275mm or 10.82 inches. So 10.82x .7 = 7.57”

Now we have our Sidewall Height. We will now multiply by x2 and get 7.57 x 2 = 15.2in’s of sidewall total.

Now add this to the rim diameter. 15.2 + 18 = 33.2 and we have a 33.2" tall tire.

So a 275/70/18 is 33x11.00R18 in High Flotation Size.

Quote:
The entire formula:
{(Sectional Width / 25.4) * Aspect Ratio} + Rim Diameter = Tire Diameter in Inches

Now that you know what all the numbers mean the hardest part is converting the Sectional Width (mm) into inches so that you may attain a High Flotation size.

I hope this write up has helped clear up some misconceptions about tire sizes and hopefully it will help you make more informed decisions when purchasing tires!



Also don’t feel bad if you were reading tire sizes wrong all of this time. I work in the tire retail industry and have high level coworkers who should know better, yet have these same misconceptions about metric tire sizes. It really is a common and BIG Problem!



I will use the next two posts below to do a FAQs and other Misc. Tire and wheel specs.

If you have any questions about any of this just ask it in the thread and I will add it and answer it to the FAQ section.

Just don’t ask me to convert your metric tire size into High Flotation. I just explained how you do that! You can do it yourself, I have faith!

For more information on understanding tire sizes:
DiscountTire.com
TireRack.com
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Old 01-29-2013, 01:28 AM   #2
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High Flotation Measuring System Explained

The most easily recognizeable and easiest to understand measuring system for tires is the High Flotation system. Every number is a measurement in inches and no converting or math is needed to read this system!

Click the image to open in full size.Source: DiscountTire.com

The first number you see underlined in Blue is the Tires Diameter in inches.

The Second number you see underlined in Red is the Tires Sectional width in inches.

The third number you see underlined in Pink is the Rim Diameter in inches.

Common Sectional Widths Converted in Inches

To convert the measured sectional width (mm) for a metric tire in inches, simply take the sectional width and divide by 25.4

235 = 9.25" wide
245 = 9.65" wide
255 = 10.03" wide
265 = 10.43" wide
275 = 10.82" wide
285 = 11.22" wide
295 = 11.61" wide
305 = 12.01" wide
315 = 12.40" wide
325 = 12.80" wide
335 = 13.18" wide
345 = 13.58" wide
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Old 01-29-2013, 01:28 AM   #3
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FAQ's - Ask me ANYTHING you aren't sure about!

1. I thought 305, 315, and 325 tires were 33", 35", and 37" tall tires respectively.
That is common, but unfortunately inccorect. And this is also insufficient information to relay what size tires you have to someone. If you just say "I have 315s on my truck" for all I know you could be talking about a 315/35/17 or 26x12.50R17 Goodyear Eagle F1 Sports car tire meant for a Corvette, or you could be talking about a 315/70/17 or 34.5x12.50R17 BFG A/T KO tire for our trucks. If you have a metric tire size you must give every number for anyone to understand what tire size you are using.

2. Should I use Nitrogen in My Tires? Is it worth it?
In my Professional Opinion, No, it's not worth it. The gains would be marginal as in a percent of a percent. Also keep in mind that 100% pure nitrogen filled tire is impossible, my best guess is you can get at most 90-95% and thats probably generous. Normal air we breath already contains 78% Nitrogen so unless a tire shop is offering free nitrogen fills, the gain is not worth it. Simply google Nitrogen in Tires and you will find many articles for and against check them out and decide if it's worth it to you.
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Old 01-29-2013, 02:01 AM   #4
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Old 03-09-2013, 02:25 PM   #5
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Lol I was going to call plagiarism, but your article is alot neater and to the point...
haha I'm jealous
http://www.f150forum.com/f31/tire-gu...rsions-151507/
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Old 03-09-2013, 07:41 PM   #6
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I didn't even see that one! I tried a search before i made this one and didn't find anything but I didn't narrow it down to this section only which is probably why I didn't see it.
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Old 03-09-2013, 07:42 PM   #7
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Quote:
Originally Posted by BassAckwards View Post
I didn't even see that one! I tried a search before i made this one and didn't find anything but I didn't narrow it down to this section only which is probably why I didn't see it.
Its all good man.. I get bored a write alot of how-to's... Glad someone took the time to make it straight to the point with relevant info. Good job
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Old 03-09-2013, 07:48 PM   #8
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Appreciate it!
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Old 04-02-2013, 11:23 PM   #9
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Now add this to the rim diameter. 19.2 + 18 = 33.2 and we have a 33.2" tall tire.


Understanding how you arrived at 33.2 by adding 19.2 + 18 has my head spinning.
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Old 04-02-2013, 11:32 PM   #10
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Quote:
Originally Posted by STEALTHY1 View Post

Understanding how you arrived at 33.2 by adding 19.2 + 18 has my head spinning.
Whoa thanks for the catch there, that should have been 15.2 + 18, it appears I messed up my original calculation which should have left me at 7.57" tall sidewall x2 = ~15.2"

I knew the answer in my head was 33.2 so thats probably why I missed it, I thought I had checked everything....
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