Your tires may not be the right ones for your car

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If you've ever looked at your tires – really looked at them – there's a slim chance you might have noticed a string of numbers, letters and backslashes on the sidewall.

If you're like most people, they make about as much sense as the glyphs on Egyptian pyramids.

For most drivers, there's no need to understand those numbers – most tire retailers can take care of the deep thinking for you.

But if you're among the growing number of people who are buying used tires or shopping for them online, understanding that string of numbers and letters could save your life.

Tire retailers say changes in the auto industry, combined with a growing online shopping culture, are resulting in more drivers buying tires that are not suited to their cars. While there are no statistics to back up that belief, there are plenty to show that ignoring one particular letter on your tires can put you and other drivers in peril.

The letter in question comes at the end of what looks like an indecipherable code on your tire's sidewall. It represents your tire's speed rating, which may be part of the problem since it means so much more than speed.

What the letter represents is how fast your tires can travel before breaking down, but is also an indication of how they brake, corner, grip and get rid of heat – all essential factors in keeping your car under control.

Changes in the auto business have also added to the confusion.

"Over the last 10 years there are more manufacturers, more tire sizes, different footprints," says Mike Butcher, senior zone manager for Kal Tire. "It's gotten a lot more complicated.

"An economy car doesn't necessarily take a low-rated tire. It's not your standard S-rated tire any more. It's a more high-performance tire built to handle the capability of the car."

The problem is many drivers simply shop by price tag, especially if they suffer sticker shock when they find that their $15,000 car needs a set of tires at $160 apiece. And if they shop on eBay or similar websites, there's nobody to tell them they may be making a big mistake.

That was shown recently during a demonstration run by Kal Tire, which took automotive writers through an obstacle course in identical cars – one equipped with the factory-recommended V-rated tire, the other with a lower-rated S tire. (The complicated rating system that seems to jump all over the alphabet adds to the confusion surrounding speed ratings. An H, for example, is two steps up from a T. See chart.)

"There's a big lack of understanding of what that tire rating means," says Melissa Arbour, senior business manager for tires and accessories at Canadian Tire. "Think of it as more of a handling rating."

The difference was shocking. Using the S-rated tire and driven by instructor Alan Sidorov, the Mazda 6 bounced through a simulated emergency lane change that left at least one passenger reaching for Gravol.

It swayed through a slalom course and laid a long track of rubber braking hard at 60 km/h and an even longer one braking at 80.

On the recommended V-rated tires, there was no roller-coaster feel on the lane change, the car whipped through the slalom like an Olympic skier and was noticeably more stable. It also came in 10C cooler, meaning it maintained better grip.

Most impressive, though, was the fact that, at 80 km/h, it stopped almost eight meters sooner.

That could be the difference between a near-collision and serious injury.

"The car's suspension and brakes are designed around that rating," says Andy Chiodo, marketing manager with Active Green & Ross tire. "Put on a lower-rated tire and your braking distance changes. Your handling changes."

What's most surprising is the difference in price between the V and S tires. At Kal Tire, it's about $20 per tire.

"Are you going to risk your life and your family's life for $80?" asks Butcher.

If you have questions about driving or car maintenance, please contact our experts at globedrive@globeandmail.com.

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