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How Are Tires Rated?

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Getting a Grip on Tire Ratings Is No Easy Task
The Bridgestone/Firestone tire recall not only brings the lethal risks of badly made tires into clear focus, it also raises the troubling question of whether consumers generally have any defense against such defects.

Tire industry experts say that American motorists are generally far too complacent about tire quality—that when buying tires, they seldom take into consideration easily obtainable information about important design characteristics.

Every tire sold in this country must comply with the Department of Transportation's Federal Motor Vehicle Safety Standard 109, as well as a separate set of standards established by the industry itself. These standards are encoded on the sidewall of every tire sold in America in the form of temperature, speed, load, traction and tread-life ratings.

To be sure, the standards are antiquated and in some cases compromised by different interpretations at different companies. Major tire manufacturers are currently trying to align European, American and Japanese standards through the TransAtlantic Business Dialogue trade policy group.

An important thing to remember is that compliance with government and industry standards is certified by the industry itself—without government oversight. At one time, federal inspectors conducted periodic audits of tires to guarantee that they met the standards stamped on their sidewalls. But even those selective inspections have long since been discontinued, according to industry officials.

Motorists' ability to trust the integrity of tire manufacturers is therefore paramount. Our lives depend on that.

Although greater international coordination and new, modern standards are needed, consumers also must be more aware of existing standards. Even the cheapest tires are supposed to meet the minimum government standards. Is that good enough?

Every tire must meet tests for temperature, traction and tread life under Standard 109. And every tire is rated for speed, load and dimension under industry standards set in this country by the Tire and Rim Assn.

Temperature ratings-designated C, B and A, with A being best-indicate a tire's ability to withstand heat, which can undermine structural integrity. The greatest threat to a tire is heat buildup.

A C rating is given if a tire can withstand a test that runs it on a machine against a steel roller at 50 mph for two hours, 75 mph for half an hour, 80 for half an hour and then 85 for yet another half-hour. The test was devised in 1968.

To achieve B or A ratings requires the tire to be tested longer and at higher speeds. The higher the rating, the greater will be the tire's ability to handle heavy loads on hot days for extended periods.

The speed rating would seem to be closely related, but it is not. The four basic ratings are designated S, T, H and Z, though there are a few others that are not commonly used.

With an S tire, the lowest rated in common use, motorists should be able to go 112 mph safely-or at least be confident that the tire will not fail at that speed, even though most of us would agree 112 mph is absurdly unsafe.

A T rating certifies the tire to 118 mph, H to 130 mph and Z to 149 mph and higher, according to industry manuals. But keep in mind that a tire rated highly for speed may carry a low temperature rating, or vice versa.

The load rating is the most difficult to understand. Every tire is rated for the amount of weight it can support, but the rating is based on an index rather than the actual weight. For example, a load rating of 92 on a typical tire for a full-size sedan corresponds to 635 kilograms (not quite 1,400 pounds). The higher the load rating, the stronger the tire. The tire's load rating should be matched to the maximum allowable load posted in a vehicle owner's manual.

Once again, tire experts say there is no relationship between the load, speed and temperature ratings.

They all can vary independently. A tire's rating reflects its materials, construction and design, says Harold Herzlich, a veteran of the tire industry who now works as a consultant. In the case of temperature, a tire's ability to shed heat actually improves as its tread wears thinner.

Traction is a nice feature, and every tire carries a traction rating of AA, A, B or C, AA being best. Ratings are based on a government skid test on a wet surface.

The tread-wear rating is supposed to tell you how long the rubber tread will last. It is based on actual driving over a set number of miles and then extrapolated to determine the life of the tire. The number is indexed to a baseline of 100. So a tread-wear rating of 200 should give you twice the life of a tire rated at 100.

So how good are the other, more critical safety standards and the information they provide motorists? There are a number of key deficiencies you should be aware of. Most important are variances in the interpretations of tests and standards by the manufacturers.

Achieving a B temperature rating should mean that every tire of a particular model should meet the minimum specification for that rating. But variances in the manufacturing process will mean that a tire designed to just meet a B rating will leave a certain percentage below the specification.

Another key problem involves variances in every vehicle's steering and suspension. Poor alignment creates more heat and wear and tear on the tire and maintaining proper alignment is the consumer's responsibility.

Of course, proper inflation is the most critical-and most often overlooked-aspect of tire safety, and that responsibility also lies with the motorist.

Improperly inflated tires are particularly dangerous when it comes to sport-utility vehicles and other trucks. An under-inflated tire on these heavier vehicles is far more dangerous than on a lighter passenger car. That's because the tire is supporting far more weight and even modest under-inflation will generate excess heat.


For more information on tire safety, visit the GM Tire Safety Web site.