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I always just wait until I get my car serviced to check the air pressure. If they got really low, wouldn’t I be able to tell by looking at them? Scott, Kelowna, B.C.

When it comes to tires, never trust the air apparent a tire that looks fine could be low enough to get you in trouble.

“Look at this tire on the outside, it looks fine but the inside is starting to break down,” said Kyle Lewarne, zone manager for Kal Tire in Langley, B.C., pointing to a handful of black dust in the bottom of a tire taken from a customer’s vehicle. “This tire may have only been driven at 10 per less cent air than it was supposed to have for just a couple of days.”

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That crumbling means the tire is getting weaker and that could lead to a blowout.

And don’t count on your car’s tire pressure monitoring system (TPMS) light to warn you. If your car has TPMS, that light only comes on when the tires are 25 per cent lower than what your car’s manufacturer recommends 27 psi instead of 36, for instance.

Tire pressure is important for safety.

BookyBuggy/istock

Deflated performance, fuel economy

Your car is designed to drive with tires at the recommended tire pressure too little air, or too much, changes how your car handles.

“An under-inflated tire results in more of the tire’s surface touching the road, which causes more rolling friction than necessary,” Lewis Smith, manager, national projects with the Canada Safety Council, said in an e-mail.

Kal Tire let journalists drive two new Ford Escapes, each with a set of brand-new tires. One Escape had tires filled to the recommended air pressure for the car, the other’s tires were under-inflated by 20 per cent.

We took both Escapes up to 60 km/h on a course with sharp turns, tight corners and sudden lane changes.

The tires looked the same (and felt the same when kicked) when parked, but on the track, the under-inflated Escape drove like it feels when you’re running through water.

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“It’s sluggish to turn and it gets road squirm,” Lewarne said. “You could be turning and the tire hasn’t caught up to you yet and you have to overcorrect.”

And it would have been worse if it had been raining. With the under-inflated tire, more of the edge of the tire hits the road.

The middle section, which siphons off water, doesn’t get as much contact as it should have and that means hydroplaning.

A 2012 U.S. National Highway Institute for Highway Safety report found that a car with tires 25 per cent below recommended pressure is three times as likely to be involved in a crash.

And that friction also means your tires wear faster that Escape at 80 per cent would get about 15,000-kilometres less tread life, Lewarne said and you’ll use more gas.

Under-inflated tires can lower gas mileage by 0.6 per cent for every one psi drop in the average pressure of all the tires, said the Tire and Rubber Association of Canada.

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Check tire pressure monthly

The Safety Council’s Smith says you should check your tire pressure at least once a month. Don’t go by the amount on the side of the tire that’s the maximum pressure the tire can handle, not the amount that your vehicle is designed for.

Look at the sticker on the driver’s side door frame or in your car’s owner’s manual to see the amount that needs to be in each tire – some cars specify different levels for front and rear.

Lewarne also suggests getting a digital tire pressure gauge instead of relying on the manual gauge at the gas station.

“The manual ones generally aren’t as accurate,” he said. “And at a gas station you don’t know the last time they calibrated it.”

Air expands as it gets warmer and that recommended pressure is for a cold tire. You should only be checking when you’ve driven less than two kilometres or it’s been sitting for at least three hours, the Tire and Rubber Association says.

“If it’s hot outside or it’s been driven all day, you could end up putting three or four more pounds of air in the tire than it needs,” Lewarne said.

Have a driving question? Send it to globedrive@globeandmail.com. Canada’s a big place, so let us know where you are so we can find the answer for your city and province.

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