How-To: Understanding Tire Numbers

Hey guys! I read "reading your tires" and that gave me an idea on putting a thread on here about... reading your tires! This took me quite some time to understand but Iv got the hang of it now. Lets get it started!

We will start out with the most common tire: the metric!

Ok guys. First off, the most important number on any tire is what follows the "R" (such as 255/75R16 or 33/12.50R16) That is what size rim the tire will fit on. I believe the R stands for Radial and the number is the size in inches of the rim. In no way, shape, or form will a certain size rim fit on another (i.e. a R15 tire will NOT fit a R16 rim, or any other size rim except a R15). Make sure you know what size rim you have before selecting a tire! This number means the same on all tires, including metric, standard, p-metric, etc etc.
From here on, im just gonna leave this number out, but it is included on all tires. This will just make things easier to understand.

Moving on to those other numbers. The first number is the width of the tire. You want to stay as close to the rims original number as possible, meaning my truck has a 255/75 tire. If you go under the number, to a 215 lets say, the tire will wear quicker on the outter edges. If you go over the number, to lets say a 285, the center will wear quicker and you may also encounter rubbing issues. Also, the wider the tire, the less mpg you get due to surface area and friction. This number can range from as low as 165 all the way up to a 325. After that, they usually turn into metric (described later). This number goes by 5s but with no zero such as 235, 245, 255, 265 etc etc. Got it?!

The middle number is the most difficult to understand. Its called the aspect ratio. Through some fancy formula (dont ask because I havent a clue lol) they get this middle number. Its essentially the height of the tire compared to the width of the tire. Meaning a 255/75 tire is taller than a 255/65 tire. This also means that a 255/75 tire is taller than a 235/75 tire. This number ranges from as low as 35 all the way up to a 85. Also, this number goes by 5s: 50, 55, 60, 65, 70 etc etc.

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So, now that you understand the "metric" way to read tires, lets move on to standard tires! These are wayyy easier to understand! These numbers read like this: 33/12.50R15. Dont get these confused with flotation tires. Those are used on tractors and such.

Anywho, standard tires are backwards to the metric tires, as in the height is the first number and the width is the second. These are easier to understand because the first number (such as 33/12.50) means your tire is 33 inches tall! Easy! This number goes by ones so that is easy too. They start as low as 29 and go up to crazy numbers like 56. You wont see that big of a number on street legal trucks lol. To better understand this, most guys will say "It had 44 inch boggers!" The height of the tire is what theyre talkin about.

The second number is width. This means a 33/12.50 tire is 12.50 inches wide! Easy! Not much to explain here. This number ALWAYS has a .50 on the end.

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Some old school stuff that you shouldnt have to worry about on a Ranger is called P-metic. These dont have an aspect ratio so they just read 185R15. You will see these on classic VWs and such.

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The last thing Ill talk about is called the load range. This number isnt super important on a Ranger just because you shouldnt be hauling 3 ton of rock in the bed lol. But just remember that the higher the number/letter, the stiffer the tires gonna be, thus the rougher the ride. This reads as either a number like 123/125 or a letter from A-E. If your not sure about the load range, look it up on the internet.

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Other stuff that you will see on the side of tires are the speed rating, tire wear, max tire PSI, number of plys, what the belts are made of and a whole load of other stuff that im not super familiar with. To make things easy, you really shouldnt have to worry about those much. You drive a Ranger so you shouldnt be driving at Mach II speeds, you shouldnt be driving through the Baja 1000, and I always use 34-36 psi. Some others can chime in here about those numbers if they want. Thanks for reading guys!

 

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I run my tires at 50 psi just like the sidewall states on my truck tires a little over the sidewall rating will also give a bit better mileage as it rolls better

 

how does the 50 psi feel when riding?

 

feels fine to me, but I have always done that with my tires and in the prius I run 40 in the rear and 42 in the front, been doing that for 5 yrs now

 

I run mine at whatever they leak out to be, just because it stops the truck from rolling on eggs.

 

I put 40 in my Stang back in the day for a little better MPG lol. Its "supposta" make your tires wear uneven though

 

Thread moved to Wheels and Tires forum, into a sticky as well

 

My first sticky! Yay! Thanks Hooligan!

 

I think you forgot the most important numbers. The age of the tire. Rubber has a shelf life and when I purchase new tires, they will be NO More than three or four months old.

Here is how you tell, the last 4 numbers on the DOT code is the date of manufacture. If the last 4 numbers are 3611, then the first two numbers tell you the tire was made in the 36th week of the year 2011 ( the 11 tells you the year). This tire should NOT be put on any vehicle today.

My tires read 1811, and was installed on my truck in the middle of June. It was installed in week 25 of 2011, a liitle less than two months old. Perfect.

Tire pressure: I have always run my tires around 45 PSI. I've never had tire wear problems.

 

The most important number is the size rim itll fit on lol. Im runnin some tires on my VW thats been sitting for at least 6 years with no problem

 

6 years is the limit by any tire dealer I have ever talked with, even if buying used tires they will toss anything older than 6yrs at least out here anyway

also rubber goes bad by not being used too, not just dry rott but rubber failure, only reason I know this my brother worked25 years at cupples company they made tires and inner tubes in MO.

 

Its just a VW though lol. I dont take it on the interstate or nothin. It barely gets drove on the highway

 

Lets help you with your tire size explination. Your example 255/75R15 is what we will use. The 255 is the section width in mm, but this is America so lets convert it to inches. Simply divide 255 by 25.4. So your 225mm section width is 10.04 inches.

Now the aspect ratio, this is a ratio of sidewall height to section width. No complicated formula needed. Your section width is 10.04 inches, multiply by 0.75 and we have the sidewall height. The sidewall height is 7.53 inches.

If your tire is a 225/60R16, then you would multiply by .60, if you have a 205/55R16 then multiply by .55, easy enough.

Now we can calculate the tire diameter, take the sidewall height times 2 plus the rim size and you get a tire diameter of 30.06 inches.

No complicated math here.

 

That is useful info! Iv always wondered how to find the "real" size of a tire. Good input!

 

to clarify the 235/75/r15 question. The 75 number is the percentage of how wide the tires is.

For example. in a 235/75/r15

the 235 is width in mm
the 75 is 75% of the width is the amount of sidewall you have
and the r 15 is the rim size.