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Getting a Grip on Tire Ratings Is No Easy Task


The Bridgestone/Firestone tire fiasco 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.

"So far, there has not been a harmonization of standards," said Jim Whiteley, vice president for global product process and quality at Goodyear Tire & Rubber Co.

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?

I asked the top tire experts at Goodyear and Michelin (let's forget Bridgestone/Firestone for the time being) whether their minimum-rated tires would enable a motorist to drive safely while carrying the maximum allowable load through the Mojave Desert on a 120-degree day at the posted speed limit of 70 mph.

Both companies said that driving on their tires under those conditions would be safe. But certainly, better tires would give a motorist a higher margin of safety. And that's important, because the chances of failure increase as a tire's maximum performance is reached.

(It's worth noting that the recalled Firestone Wilderness AT model equipped on the Ford Explorer carries the lowest possible ratings for both speed and temperature, while it has a middle-range rating for traction.)


So let's review those standards, which, unfortunately, can be difficult or nearly impossible to understand.

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 biggest enemy of tires is heat buildup," Whiteley said.

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, according to Mike Wischhusen, director of product marketing at Michelin. 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.

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