This is the final article on automobile tires by John Thomson.

As we have seen, tire manufacturers have a number of tire sidewall markings that set forth the characteristics of each particular tire. In this last article we will discuss the manufacturers’ labeling conventions for speed (how fast the tire is designed to go) and load (how much weight the tire is designed to bear). Both factors must be considered when selecting the appropriate tires for an automobile.

Let’s look at how speed affects tires. As the speed of a tire is increased, its components tend to "grow" or deform due to centrifugal force. This deformation leads to heat generation. Basic physics holds that the faster a tire spins, the more it deforms, and the more heat is generated—it’s a natural phenomenon. Heat is one of the major contributors to tire failure, especially at high speeds.

Tire failure at high speeds is usually referred to as "catastrophic" tire failure because it usually leads to loss of control and a serious accident. Therefore, controlling heat levels is important for tires driven at high speeds. To overcome this problem, speed-rated tires are designed with advanced belt configurations engineered to resist and control deformation under speed so that the integrity of the tire is maintained. By engineering them to experience less and less deformation, tires may be driven at correspondingly higher speeds.

A speed rating is a letter designation on a tire’s sidewall that indicates the maximum speed capability of that tire. It’s a measure of the soundness of the tire at high speeds. Speed ratings were originally developed in Europe to help owners of high performance sports cars choose replacement tires that would match the capabilities of their cars. A wide range of speed ratings was made available, and car manufacturers came to specify particular ratings for particular cars. As a result, those speed ratings are at times incorrectly associated with the handling capabilities of a car. For North America, speed ratings have generally been condensed into six ratings. We will look at these ratings next.

The speed rating of an automobile tire, sometimes referred to as the speed rating index, is an alphabetic indicator of the maximum vehicle speed for which the tire manufacturer has designed a tire. The overall speed rating system for all vehicles sets forth speed ratings from B (31 mph) to Z (over 150 mph). Passenger car tire speed ratings generally range from S to Z within that system and are set forth in the accompanying speed symbol chart. The speed rating usually appears at the end of the letters and numbers indicating the tire size on the tire sidewall. For example, P275/40R17 93H denotes a tire rated for speeds up to 130 mph, and P275/40ZR17 93V signifies a tire rated for up to 150 mph. There are some intermediate and secondary speed notations in use by some manufacturers, but they are beyond the scope of this article.

Speed ratings are developed using standardized indoor wheel tests similar to those used by NHTSA for temperature ratings. In the case of speed ratings, the tire is subjected to an indoor wheel test prescribed by the Procedure for Load/Speed Performance Tests of the Economic Commission for Europe (ECE-30). The properly inflated tire is subjected to the load for which it is rated (see below) and run on a 1.7 meter wheel. (See the accompanying picture, which shows a tire being tested.) A tire must survive at least 30 minutes at speed to earn the commensurate speed rating.

Remember that a speed rating is the speed at which a tire design has been rated for thirty minutes of running at that speed. Good tire design does not repeal the Law of Physics—running a set of tires at or above its rated speed for a long time courts tire failure and invites a fatal accident. You are more likely to stay out of trouble if you buy tires that have speed ratings that exceed the demands of your driving habits. Replacement tires should have the same or higher load and speed ratings as the original equipment tires specified by the vehicle’s manufacturer.

Maintaining the correct tire pressure is important, since a tire’s speed rating is valid only when it is properly inflated. Underinflation causes heat buildup, the enemy of tires. Repairs affect a tire’s speed rating since any tire’s stated speed rating is invalidated by any repair. According to tire manufacturers, the maximum speed for an otherwise speed-rated tire after it has been repaired is no more than an unrated tire, that is, 85 miles per hour.

Now let’s look at what is meant by the load rating of a tire, later known as its load index. You will recall from my previous article that the Alpha-Numeric tire sizing system uses an alphabetic "load rating" scheme. The use of load ratings on Alpha-Numeric tires replaces the previous Numeric method where the number of plys a tire has is an indicator of how much weight it can handle. In the Alpha-Numeric load index system, the load for an alpha size is the same regardless of rim diameter or aspect ratio. For example, sizes F70-15, F78-14, and FR60-13 all have a maximum load of 1500 pounds when inflated to 32 psi. Generally the manufacturer of an automobile will specify a tire load rating for the tires to be used on the car.

The load index for tires using the later Metric tire sizing system is a numeric indicator, which can be found on a tire’s sidewall following the size description and just in front of the speed rating of the tire. Though the overall system contemplates load indices from 65 (639 pounds) to 104 (1984 pounds), passenger car load ratings range generally from 75 (852 pounds) to 100 (1764 pounds). Therefore a tire marked 205/60R15 89H is rated to carry 1279 pounds (denoted by the 89) and speed rated at 130 mph (denoted by the H). Remember that all tire load ratings are valid only when the tire is fully inflated to the recommended pressure. As we have seen, underinflated tires generate destructive heat that leads to tire failure.

Factors other than those discussed above have a detrimental effect on a tire’s originally specified speed and load ratings. Exposure to sunlight and ozone breaks down the chemical composition of tires, as does grease and gasoline. Tires exposed to these materials during driving and storage may be weakened and subject to sudden failure, regardless of their original speed and load designation. When tires are stored, they should be stored in a cool, dry place away from these materials. Be sure to allow air to circulate around all side of the tires, including underneath, to prevent moisture damage. Don’t store tires on black asphalt, other heat-absorbing surfaces, snow-covered ground, or sand.

The material for this series of tire articles was drawn from many sources, most of which are to be found on the World Wide Web. I particularly want to thank Joe Pacuit of the Tire and Rim Association for his assistance in obtaining some hard to get information.