Tire Rating Definition

It is difficult to overstate the importance of automobile tires. Tires figure prominently in a car's safety, performance and efficiency. To make good comparisons across the different brands and styles of tires available, a series of ratings exist. The purpose of these ratings is to allow drivers to choose the best tires for their vehicle and to be able to properly maintain those tires once they have been chosen.

Load and Pressure Ratings

All passenger and commercial vehicles sold in the United States come with tire load and pressure placards. These labels are usually stickers that are attached to a vehicle's door frame, glove box or below the dash. Tire placards contain some of the most basic and important information about the vehicle's tires, including the manufacturer's recommended tire size, the recommended inflation pressure in pounds per square inch, and the gross vehicle weight rating (GVWR). The GVWR is the maximum weight of both passengers and cargo that the vehicle is designed to carry safely, and is in large part a function of the tires.
It is important that a driver adhere to maximum loads and monitor tire pressure frequently with adjustments as necessary. Under-inflated tires not only decrease a car's fuel efficiency, but they also increase the risk of poor handling and loss of control.

Tire Size

Cars will only perform to manufacturer specifications when they are fitted with tires of the proper size. Tires must match one another and all must fall within the range of sizes that ensures accurate gauge readings (even under-inflated tires of the proper size can lead to incorrect speedometer readings).
Tire size can be identified by the rating printed on the sidewall of a tire. For passenger cars, this rating begins with the letter "P". (Light truck tires receive a "LT" designation.) The next number in the sequence represents the width of the tire in millimeters. This number is followed by a slash mark (/) and then another number which represents the ratio of the tire's height to its width. This factor is known as the tire's ratio. After the ratio a letter "R" stands for a radial style tire. The final number in the rating is the diameter of the wheel onto which the tire will fit in inches.

Other Ratings

The sidewall of a tire contains other important information as well. Temperature ratings indicate how resistant the tire is to sustained operation at high temperatures. This scale ranges from "A" for the most heat resistant tires, to "C" for the lowest rating to still qualify as safe.
A tire's traction rating is also indicated with a latter. The top of this scale is the "AA" rating, indicating a tire with superior ability to stop on wet surfaces. "A", "B" and "C" traction grades follow.
The third rating listed alongside temperature and traction grades is the rating for tread wear. This number is a percentage of the tire's expected lifespan compared to a "standard" control tire used for testing purposes. Typically this rating falls between 300 and 500, indicating tires that are expected to last between 3 and 5 times as long as the control tire.

Load and Speed Ratings

Although load ratings appear alongside tire pressure ratings on a vehicle's load and pressure placard, a load rating is also assigned to each tire. This rating is represented as a number known as the load index. Load index charts are needed to determine a tire's maximum load in pounds or kilograms. Load range is yet another load rating, this time represented as a letter between "A" and "N". These letters correspond to a series of "ply ratings" which have no standard definition but are useful in comparing tires manufactured by the same company.
Speed ratings are represented by a non-continuous series of letters. Each letter corresponds to a maximum speed at which the tire is expected to perform safely, though driving conditions and the age of the tire affect maximum safe speed as well. The lowest speed rating, "M", refers to a maximum safe speed of 81 miles per hour. The highest speed rating is "Y", which corresponds to 186 miles per hour.

Intended Use Ratings

The majority of tires are also designated as falling within one of the unofficial use categories. These categories are functions of tread pattern and the material composition of the tire. High Performance tires feature softer rubber compounds (and therefore a shorter tread life) and are intended for use on dry pavement. Mud and snow (M&S) tires are designed for frequent use in winter conditions. These tires may perform well on dry pavement but also feature wider tread patterns for superior traction on icy roads or in deep snow. All Season tires are essentially a less extreme version of M&S tires, and are designed for occasional use in winter conditions. Finally, All Terrain tires are designed for off-road use and feature the most dramatic tread patterns.

References

• National Highway Traffic Safety Administration
• Consumer Reports

Resources

• Tire Reviews and Ratings