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Writing On The Wall - Sidewall Reading

Posted in How To on April 1, 2013
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Photographers: Courtesy of the Manufacturer

Have you ever wondered what all of those extra numbers and letters on your tire’s sidewalls really mean? Well, wonder no more! Here we’ve decoded the most common sidewall jargon plastered alongside your 4x4’s tires. From speed ratings and plies to load capacities and build dates, your tire’s sidewalls are full of important and interesting information. We’ve even provided you with a somewhat easy-to-use formula for deciphering the cryptic coding on metric tires. Now if we can only figure out how to make a self-healing tire that doesn’t wear out, we’ll have it made.

Load Range
The tire’s load range is represented by a letter ranging from A to N. Before the introduction of the radial tire, the load range letter correlated to the number of plies in the tire’s construction—A being two, B four, C six, and so on (always increasing by two per letter). It now has more to do with the ply-strength equivalent. While there is some correlation, there is a difference between load rating and load range. Just remember that load rating determines the exact weight that the tire can support. The load range has more to do with the tire’s construction.

Max Tire Inflation
This one is pretty obvious. It represents the max amount of air that the tire is designed to hold when cold. The air pressure inside of your tire also translates directly to the tire’s performance and load holding capabilities. Given that as you travel down the road the tire heats up and expands, it’s always suggested that you check the air in your tire once it has cooled off or sat for an appropriate amount of time. Elevation and temperature can affect the tire’s psi.

Ply Label
The tire’s ply label represents the amount of plies and the materials used in construction. Plies are the interlaced material that make up the DNA of the tire. Different manufacturers use proprietary formulas for any given tire. It can be a combination of polyester, nylon, Kevlar, and steel. More plies can equate to a more durable tire, but that can also mean the tire is stiffer.

As we mentioned earlier, plies correlate with the load range letter on the tire. While a load range D tire is equipped with an 8-ply rating, it no longer means that the tire has 8 plies, only that the plies used are of the equivalent strength. The number of plies as a unit of measure dates back to when nearly all tires were bias.

Load Index and Speed Symbol
The load index number assigned to a given tire represents a numerical value that correlates with the tire’s load carrying capacity—the higher the number, the higher the load index. For example; a load index of 85 equates to 1,135 pounds, while a load index of 121 (shown here) is 3,197 pounds. Matched with the load index number will be a marking that indicates the speed rating of the tire.

Speed ratings are meant to match the ability of the tire with the vehicle that it is on. You wouldn’t want a 106 mph speed rated R tire mounted on your Ferrari. For the most part, the farther along the alphabet, the higher the speed rating, but there are a few exceptions. While a Z-rated tire used to be thought of as the highest speed rating, tire manufacturers now have W- and Y-ratings that go beyond the original 149 mph Z-rating.

LT and P
P-Metric, European Metric, LT-Metric, and Light Truck are all indicators of the tire’s type and associated category. The designation of a P or LT in front of a tire’s sizing is most common and represents if it is a passenger car (P) or light truck (LT) tire. For our use, most tires fall into the LT category. (Note: The M+S you see on the tire pictured stands for mud and snow.)

Most tires will have some badging that identifies if it is of radial or bias construction. Radial is usually represented by an R placed next to the tire’s size or spelled out along the sidewall. Radials make up the majority of the tire market. We’ll get more into the pros and cons of bias and radial construction elsewhere in this issue.

Tire Height Formula

Section width x aspect ratio=A mm

A mm x 2= B mm

B mm /25.4 = C inches

C + rim size = tire height in inches

(Tire width formula)
Section width /25.4 = width in inches

Metric Formula
If you drive to any given auto dealership, chances are you’ll find the majority of the tires sidewalls will be equipped with a metric label. This could be something along the lines of a 265/70R17, for example. To find out the actual size of the tire, there is a formula. The 265 millimeter number represents the tires section-width, while 70 marks the tire’s aspect ratio.

The smaller number is a percentage of the tires width. In this case the width is 265mm and the height is 70 percent of that. To calculate this you multiple 265 x 0.70—this nets you 185.5mm. To get the tire’s overall height, you double the 185.5, which gives you 371mm. To convert this number to inches you divided the 371by 25.4. This leaves you with 14.60 inches.

The final step is to add 14.60 to the rim’s diameter (in this case 17 inches). This means the 265/70R17 is roughly a 31½-inch tall tire (31.6) that’s a touch over 10 inches wide (10.43). The simple equation to remember is that any given millimeter number can be divided by 25.4 to give you that number in inches. If you really want to avoid the math headache, you can just use one of the many conversion calculators online. But where’s the fun in that?

When you see a tire marked with an easy to read size such as a 37x12.50R17, it is called a flotation tire. Unlike the mind-stumping math-fiasco that is the metric tire system, flotation sizing puts everything in an easy-to-read inches formula. For our example of a 37x12.50R17, the 37 represents the outside diameter, the 12.50 marks the section width (not to be confused with tread width), R stands for Radial, and 17 is the rim diameter.

DOT Safety Code
Similar to your favorite cold beverage, tires have dates stamped on the outside. Since 2000 all tires must have the week and year of manufacture carved on the sidewall. This four digit number can usually be found near the DOT stamping, which is often set close to the bead of the tire. A code of 3612 would mean that the tire was built on the 36th week of 2012.

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