You wouldn’t put golf-cart tires on a performance car or airplane tires on a dump truck.
But that’s the effect you might get, in terms of handling, comfort and engineering, if you buy the wrong tires for your car by overlooking two critical features: speed ratings and load indexes.
Speed ratings indicate how fast a tire is certified to run at long stretches under laboratory conditions. The ratings range from B (31 m.p.h.) to Y (188 m.p.h.). Most passenger tires are certified for speeds of 99 to 118 m.p.h.
Load indexes determine each tire’s maximum load-carrying capacity. The index runs ranges from 65 (639 pounds) to 125 (3,690 pounds).
You can find both of these ratings on the tire sidewalls, at the end of the long string of numbers and letters that indicates the tire size.
It will read like this: 82H. In this case, the 82 refers to the load index (each tire can carry 1,047 pounds) and the H refers to the speed rating (maximum certified speed of 130 m.p.h.).
But the numbers can be misleading. You probably never drive faster than 70 or 80 m.p.h., even in the passing lane of an interstate with your radar detector turned on.
Does that mean you’re safe with a cheaper tire certified for 65 or 80 m.p.h.?
No, tire experts say, for three reasons: First, a particular car is designed with a particular tire in mind. If you buy cheaper tires for an expensive car, you might not get the proper handling even at low speeds (though you could get a trade-off benefit, such as better comfort, because the tire is built to be less rigid, to take corners better at high speeds).
“The overall vehicle is designed as a system,” said Mike Wischhusen, director of product marketing for Michelin North America. “The vehicle has a maximum speed capability, and all the components, including the tires, have to be capable of that.”
Second, you never know when you’ll have to go faster than 65 or 70 m.p.h., even if you do most of your driving on city streets.
Third, if you don’t keep your tires properly inflated or otherwise abuse them, the speed ratings become invalid. Your tire could begin to overheat, leading to possible tread separation or other failure.
The flip side of this question: Can you safely travel faster by putting on higher speed-rated tires than your car calls for? No again.
The ratings apply only to the tires, not the car. Other systems in the car could begin to break down at those speeds or put undue stress on the tires.
So, what happens if you drive at 120 m.p.h. for long stretches on a tire certified for only 99 m.p.h.? Will it blow up?
In the laboratory, it probably will, after a certain amount of time. On the road, it’s hard to say how much punishment a tire can take.
“I don’t think anyone can say a tire will self-destruct at a few miles faster than the rated speed,” Wischhusen said. “But it pays to be safe.”
