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Who Taught You to Drive

Sensing a problem

Low tire-pressure warning messages should be taken seriously to avoid damage to tires.

Peter DeMarco

Low tire-pressure warning messages should be taken seriously to avoid damage to tires.

On a clear Saturday morning, it takes about 20 minutes to drive to South Station from my apartment. So to be safe, we gave ourselves 35 minutes — plenty of time, I figured, for my fiancée to make her bus to Saratoga Springs, N.Y.

As soon as we got in the car, though, I knew we were in trouble.

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“Tire Pressure Low” warned my dashboard screen. Three of my tires had normal readings of about 35 pounds per square inch (psi), but my front driver’s-side tire registered just 17 psi, half of what it should have been.

To my eye, the tire did not look that flat — actually, it looked quite normal — so we drove to the nearest gas station for a quick top-off of air. But no matter how much air I pumped, the tire’s ressure, bizarrely, would not go up.

The dashboard screen remained fixed at 17 psi — and we were running out of time.

“We’ve got to go!” Laura yelled from inside the car.

My own blood pressure soared. Could I drive 5 miles on Interstate 93 with a deflated tire? If the tire lost more air, would I be able to control the vehicle?

If I had a leak, shouldn’t I at least have heard a hiss?

Flat, or not?

Before I tell you the rest of the story, we really should to go back to my very first question: If my tire was that low, shouldn’t it have looked flat?

Not necessarily, said John Paul, AAA of Southern New England’s “Car Doctor.”

Truth is, with most modern tires, it is often difficult to tell a flat tire from one that is filled, in part because the sidewalls of most radial tires are constructed with a slight bulge built into them, Paul said.

“Radial tires always kind of have this curve, so they always look squishy,” he said.

Radial sidewalls are also very strong, so they retain their shape for a long time, even while losing air.

“When I worked in a garage or did AAA clinics, we’d have some people come in with tires at 12 psi, and they didn’t even notice. And that’s at a critical level for a tire,” Paul said.

In the end, we did risk the ride to South Station, making it there, on time, without incident. But was I foolish to drive on a tire that was only 50 percent inflated?

Paul did not think so, given how I drove very slowly on the highway, no faster than 35 miles per hour, and babied the car on turns and stops. But had the tire pressure dropped lower, I would have been flirting with disaster.

“When a tire is low enough, the sidewall will [finally] expand and will actually be rubbing on the road when you’re driving,” Paul said. “People all the time say, ‘The car was fine, and all of a sudden, I had a blowout.’ Well, the tire was not fine. The tire was so low the sidewall was hitting the pavement, causing it to fail.”

Even if you escape a blowout, if a tire’s sidewall scrapes the road, that tire is never going to be the same, said Matt Edmonds, vice president of TireRack.com, a national online tire retailer that also independently tests hundreds of models of tires.

“Once a tire gets down to that point, more heat builds up in the tire, damaging the inside construction of the tire, which is damage you can’t see,” Edmonds said. “At that point, you’re more apt to blow out from hitting a pothole or something later on.”

The plot thickens

Having made it to South Station, I pulled into another gas station on the way home. This time, the air pump hose had a manual tire pressure gauge attached to it, and to my surprise, the manual gauge said I had too much air in my tire — a full 50 psi.

I scratched my head. Which gauge was right?

Again, let’s take a step back.

My car, a Chevy Equinox, has a “direct” tire pressure-monitoring system. With direct systems, air pressure in each tire is measured by a finger-long sensor chip affixed to the valve stem that pokes into the tire. The sensor is in the hollow of the tire, reporting readings to your car’s on-board computer via a radio wave.

Yep, your car’s tires essentially have their own Wi-Fi.

“You just can’t hard-wire into that area of the vehicle,” Edmonds said.

“Indirect systems,” by contrast, rely on sensors that track how fast your tires are rotating. When a tire loses air, its diameter shrinks, allowing it to complete each revolution just a tiny bit faster. When the car’s computer discovers a tire rotating faster than it should, it flashes a warning.

Direct systems are more user-friendly because they tell drivers which tire is in trouble, and its exact pressure — typically, a drop of 5 to 6 psi — triggers your first warning. However, because a direct system’s sensors are not hard-wired, they must run on batteries, which typically die after 5 to 7 years.

“You can spend $100 or more to have the tire taken off, the sensor taken out, and the new sensor programmed to match your car,” Paul said.

Indirect systems flash a general warning message, without telling you which tire is low, or its pressure reading.

Faulty gauge?

Did I have too much air in my tire?

Both Paul and Edmonds said that manual gauges, in general, are very reliable — so much so that each expert recommends carrying one in your car (some are as cheap as a few dollars) and using it to check all your tires at least once a month, regardless of whether you have a fancy, on-board monitoring system.

After letting some air out of my problem tire, my car’s dashboard reading returned to a more normal 35 psi, proving to me that it was, indeed, overinflated.

A few days later, I brought my car to my Chevy dealership, and the tire passed a thorough inspection, including its tread, sidewalls, and the valve stem. The way I figured it, I never had a flat tire at all; the sensor must have temporarily malfunctioned.

But my tale, Edmonds warned, is not typical.

Sensors are accurate far more often than not. If a screw or nail becomes lodged deeply enough in a tire, it might temporarily plug its own leak. But hit one good bump, and you will be losing air again.

“It’s incredible the number of people who ignore the warnings,” Edmonds said. “I came home one day and said to my wife, ‘Your left, rear tire is flat.’ She said, ‘You know, that light on the dashboard did come on.’ She’d driven 80 miles that day, and destroyed the tire.”

Peter DeMarco can be reached at peter.demarco@globe.com. His Facebook page is “Who Taught You to Drive?” and on Twitter @whotaughtU2driv.
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