Being able to read and comprehend the information printed on a tire's sidewall will make it easier for you to understand your tires and assist you in choosing a replacement set.

Numeric Sizing System
This first system developed for tire sizing was used until the late 1960s, but provided only the cross section width of the tire and the rim diameter in inches. If the section width ended in zero (e.g., 7.00-14 or 7.50-14), the tire had a common aspect ratio of about 92. For section widths not ending in zero (e.g., 8.25-15), the tire was considered "low profile" with an aspect ratio of about 82.

Alpha-Numeric Sizing System
In 1968, a new concept was introduced worldwide. The Alpha-Numeric sizing system is a load-based system where tires are designated by their load-carrying capacity and aspect ratio. The first letter is the load and size relationship, with letters ranging from A to N. The lower the letter, the smaller the size and, of course, the lower the load-carrying capacity of the tire.

P-Metric Sizing System
To accommodate the smaller tires used on compact cars, the P-Metric (Passenger Metric) system was created in 1976. The maximum inflation pressures of P-Metric tires were raised for lower rolling resistance. The P-Metric system is widely used by domestic tire manufacturers.

ISO Metric Sizing System
The International Standards Organization (ISO) Metric system combines the Metric system with a service description. The service description provides the load index along with the speed rating symbol.

Millimetric Sizing System
The Millimetric sizing system is similar to the Metric system except that the rim diameter is also represented in millimeters.

Light Truck Numeric System
Similar to the Numeric system for cars, it lists the section width in inches, construction type, rim diameter in inches, plus the light truck designation.

Light Truck Metric Sizing System
Similar to the P-Metric system, except the P is replaced with the LT light truck designation. Also, LT-Metric and P-Metric tires differ in construction.

Light Truck High Flotation System
The same as the Light Truck Numeric system with tire diameter added to the front.

Often referred to as the profile or series, the aspect ratio of a tire is determined by dividing a tire's section height by its section width when the tire is: Inflated to maximum air pressure, Mounted on the approved measuring rim, and Under no load.

The lower aspect ratio responds to lateral force more effectively than a tire with a higher aspect ratio.

The aspect ratio affects steering stability. Generally, the shorter the sidewall, or the lower the aspect ratio, the less time it takes to transmit the steering input from the wheel to the tread. The result is quicker steering response.

Aspect ratio also affects the tread contact patch. As a rule, a low profile tire produces a wider tread contact patch. This wider tread contact patch creates a stiffer footprint that reduces distortion and provides improved cornering traction. Aspect ratio also impacts ride. A low profile tire usually has a stiffer ride than the standard aspect ratio of 75 or more.

Service Description
Many tires come with a service description added on to the end of the tire's size. These service descriptions contain a two-digit number (load index) and a letter (speed rating). The load index is a representation of the maximum load each tire is designed to support. Because the maximum tire load capacity is branded on the tire's sidewall, the load index is used as a quick reference.




Use the following chart to determine the maximum load-carrying capacity based on a tire's load index:

Speed Rating
Speed ratings are certified maximum sustained speed designations assigned to passenger car radials and high performance tires. Because of the evolution of high-speed passenger car travel, it was necessary to establish a way to rate a tire's high-speed capability. In the U.S., these ratings are based on tire testing in laboratory conditions under simulated loads (European testing uses actual road testing). For a tire to be speed rated by the U.S. Government, it must meet certain minimum government standards for reaching and sustaining that specified speed. Domestically, high performance tires must be speed rated. The tire industry defines high performance tires as those with speed symbols of "S" or greater and aspect ratios of 70 or lower. Yokohama goes one step further and defines high performance tires with a speed symbol of "H, V, W, Y," or "Z" and an aspect ratio of 70 or less (typically, 60 or less). Conventional passenger car radials need only meet the minimum Department of Transportation standard of 85 mph. Speed symbols may currently be marked on a tire in any of three ways: 205/60ZR15; 205/60ZR15 89W; or 205/60R15 89W. The International Standard Organization system (ISO) currently serves as a worldwide standard for tire markings. At the end of a transition period, any speed symbol denoting a fixed maximum speed capability will be at the end of the service description following the tire marking (illustrated in the second and third examples above).

Warning:
Speed ratings apply only to the tire, not to the vehicle. Putting a speed rated tire on any car does not mean the car can be operated at the tire's rated speed.

Ply Rating vs. Load Range
Ply ratings and load ranges identify load and inflation limits of a given tire size when used in a specific type of service.

• Ply ratings: An older method of rating load capacity, these are listed as 4-ply, 6-ply, 8-ply, etc.
• Load ratings: The current method of rating a tire's load-carrying capacity is denoted by letters (B, C, D, E, etc.).

Uniform Tire Quality Grade (UTQG) Labeling
Required by the government, the UTQG provides comparative manufacturer information. Tires are subjected to a series of government-mandated tests that measure performance in treadwear, traction and temperature resistance. All testing is done by the tire manufacturer.

Treadwear
Treadwear is a measurement of tread durability. Tested against an industry standard, the assigned numerical grade indicates how well the tread lasts compared with a reference standard of 100. A treadwear rating 200 means the tread wears twice as well as the standard. Actual wear depends on the conditions under which the tire is used. Driving habits, service practices, differences in road surface and varying climates all affect treadwear.



Traction
Traction is a measurement of a tire's ability to stop on wet test surfaces of asphalt and concrete under controlled conditions. Traction grades are assigned by the UTQG system and branded on the sidewall. Traction grade is determined only for straight-ahead, wet braking on concrete and asphalt. It doesn't include cornering, which may also be an important customer performance need.

• Traction Grade A: The tire performed well on both surfaces.
• Traction Grade B: The tire performed well on at least one of the surfaces.
• Traction Grade C: The tire performed poorly on one or both of the surfaces.

Temperature Resistance
The UTQG also provides a measure of resistance to heat generation under normal operating conditions. The test is conducted under predetermined standards for inflation and loading. Excessive speed, underinflation and overloading can all cause adverse heat build-up. Sustained high temperatures can reduce tire durability. Resistance grades are branded on the sidewall.

• Resistance Grade A: The maximum performance level indicating the tire withstood a half-hour run at 115 mph without failing.
• Resistance Grade B: The tire passed 100 mph but not 115 mph.
• Resistance Grade C: The minimum performance level indicating that the tire failed to complete a half-hour at 100 mph.

Department of Transportation (DOT) Certification
"DOT" is branded on the tire's sidewall indicating the tire is certified by the Department of Transportation. Following the DOT branding is a serial number designating the tire manufacturer, manufacturing plant, tire size and date of manufacture. Federal law requires that tire dealers record the DOT identification numbers along with the tire buyer's name and address.




Additional Tire Labeling Conventions Mud and Snow Labeling
If a tire is rated for safe performance in mud and snow, it will be noted on the sidewall of the tire with either M/S, M+S or M&S. A tire is certified under the definitions set forth by the Rubber Manufacturers Association (RMA).




Tire Construction Labeling
Tread ply and sidewall ply information, including tire ply composition and materials used, must also be identified. An example would be: Tread: 2 Plies Rayon + 4 Plies Fiberglass Sidewall: 2 Plies Rayon