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I purchased a 2013 Lincoln MKX in January 2013. It currently has 12,500 km. I noticed a cut on the sidewall of a tire and would like to have it replaced. However, I am being told that I should replace tires in pairs. Someone else told me I would have to replace all the tires or the AWD would not operate properly. Before I go to the tire store, could you please advise me if I have to replace one, two or all four tires. – Martin, Toronto

Can you ease into re-tirement by changing one tire at a time? Probably not – it depends on how much wear you have on the other three, says the Rubber Association of Canada.

"If you had a brand new car, had only driven it for a couple of days and were replacing it with the same kind of tire, I'd say you could replace just the one, no problem," says Ralph Warner, director of operations for the association, which represents Canada's tire manufacturers. "But without seeing them, it's impossible to judge."

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Just a few thousand kilometers on the road could wear down the old tires enough that a new tire – even if you managed to find the exact same model as the others – could have noticeably deeper treads. A good tire shop would be able to tell you if the difference is subtle enough to let you replace just one.

"There's no firm and fast rules, it's a judgment call," Warner says.

Here's why. Because it still has full treads, the new unworn tire would be larger than the rest, which could make the traction control system (TCS) think the other tires are spinning.

"Traction control systems sense revolution of the tires and compare one to another, they respond when they find one tire is slipping," Warner says. "You run the risk of tricking the TCS into acting when in fact there's nothing wrong."

The TCS and Anti-lock Braking System (ABS) use speed sensors in each wheel. The system doesn't know that one of the tires is a different size. It just knows that the tires are spinning at different speeds, and that it needs to get them to spin at the same speed. Most systems do that by triggering the brakes.

Often, you can replace two tires instead of all four. With the MKX, Ford says it's usually fine to replace tires in pairs, since the ABS and TCS can compensate if the front and rear tires aren't worn equally.

Christine Hollander, Ford of Canada's communications manager says the AWD system isn't the problem here - it shouldn't be fooled into activating when it doesn't need to by tire wear alone. But the replacement tires should be the same size, and have the same speed rating and load index as the others.

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While changing two is fine, four new tires guarantee that all the tires start out with the same amount of traction – and traction keeps your car in the road, Warner says.

If you combine two badly worn tires with two new tires, you could get the same trouble you'd get if you put two winter tires on a car instead of four.

With the stickier tires on the front and all seasons on the back, you could get oversteer, meaning the car turns more sharply than you want it to, which could steer you into an oncoming vehicle. If the new tires are switched to the back, you could get understeer, which is when the car doesn't steer enough. You could drive straight though a corner instead of turning.

Understeer is the lesser of the two evils (there aren't usually oncoming Mack trucks in ditches). If you do get just two new tires, make sure the newer tires are on the back, Warner says.

"Bottom line, find a tire shop you trust and get their advice," he says. "I know people get concerned about whether the service guy is trying to just get money out of them... but your car's designed to have equal traction on all four wheels."

*****

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Send your automotive maintenance and repair questions to globedrive@globeandmail.com

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