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Sidewall Numbers and Letters Tell a Tale about Your Tires

Taking a close look at your tire’s sidewall can provide you with just about everything you really need to know about your tires. But, before diving into the language of tires, let’s consider what type of tires you have. Automobile tires generally fall into the following major categories:

P-Metric – P-metric sized tires are the ones with the “P” at the beginning of the tire size, such as P206/60R15. Their load capacity is based on an engineering formula which takes into account their physical size and the amount of air pressure.

European Metric- Euro-metric and P-metric tires in the same size, such as P206/60R15 and 206/60R15, are equivalent in their dimensions with only slight differences in their load capacity and inflation pressure. They are interchangeable if used in axle pairs or sets of four.

LT-Metric– LT-metric tires are equivalent to P-Metric tires, but have slight changes load abilities. Many 1/4-ton and 1/2-ton vehicles use Euro-metric and P-metric sized tires, while 3/4-ton and 1-ton vehicles use LT-metric sized tires.

Light Truck High Flotation – This designates tires with lower aspect ratio and higher flotation characteristics, which provide improved traction off-road. Information on these tires is always stated in inches.

T – Temporary Spare – If the lettering on a tire’s sidewall begins with a “T,” it signifies that it’s a Temporary Spare designed to be used only a limited time until a flat tire can be repaired or replaced.

The set of numbers and letters on your tire’s sidewall, 206/60R15 for example, can provide you a wealth of information. The first number represents the Section Width or the width of the tire. This is the distance between the outsides of the sidewalls of the tire, measured in millimeters or in inches on a high flotation tire.

The next number is the Aspect Ratio. This is the ratio between the tires Section Height, which is the distance from the rim seat to the outside of the tread, and the Section Width. The Aspect Ratio is typically expressed in increments of 5, such as 60, 70, and 75.  So, on a 206/60/15 tire, the Aspect Ratio is 60% of the section width.

Another piece of information is the tire type.  There are two basic types of tires:
* R –Radial: A radial tire is one with supporting belts, usually made of steel, running radially around the tire.
* B – Bias: A bias tire (or cross-ply tire) is one with plies that run diagonally from one bead to the other. One ply is set on a bias in one direction, and then succeeding plies are set alternately in opposing directions and cross each other.

The final number in the size of a tire is the Rim Diameter. Tires that have a rim diameter expressed in inches are the most common type used. Tires and wheels that have a rim diameter expressed in half inches (14.5, 15.5, 17.5 and 19.5) are used on some heavy-duty trailers, trucks and box vans.

You can also find important information about your tires by looking at the Unified Tire Quality Grading (UTQG) UTQG lettering. The (UTQG) system measures three things:

Treadwear (520)
This rating indicates a tire’s relative durability, based on government standard testing after about 7k miles of driving on a controlled test course. The rule of thumb is that you can add two zero’s to the number to estimate the average service life you can expect to get from the tire under normal driving conditions.

Traction (A)
This rating is based on government tests to measure a tire’s ability to stop a car moving straight ahead, on both concrete and asphalt surfaces. Tires are assigned traction grades of AA, A, B or C, with AA being the highest.

Temperature (A)
The temperature rating indicates a tire’s relative ability to withstand high operating conditions. Tires are assigned temperature grades of A, B or C based upon testing with A being the highest.

The DOT Number on your tire’s side wall also includes important information.
* The first two characters designate the tire manufacturer and plant code.
* Characters three and four denote the tire size.
* The fifth, sixth, seventh, and eighth (optional) characters identify the brand as well as other characteristics important to the manufacturer.
* The final four numbers denote the date the tire was produced.

Information contained in the tire’s Service Description is the tire’s load index and speed rating. The Load Index is an important number regarding towing or hauling. Load index numbers range from 75 (load carrying capacity of 852 pounds per tire) to 125 (3638 pounds per tire).  The Speed Rating indicates how the tire is rated for its capacity to perform at high speeds as tested by the U.S. Government and is represented by letters Q through Z. In general, a higher speed rating will result in better overall handling.

Remember, your tires are your only connection between you and the road. Now that you know the language of tires, remember to treat them well.


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