Tire pressure monitors give measure of safety

It took one flat tire to convince Marshall Sumner that his car needed tire pressure monitors as an early warning system.

More than a year ago Sumner, a computer programmer, was cruising on a crowded Southern California highway when his Cadillac Catera began making loud thumping noises. First, he thought the transmission had broken but then realized a rear tire had gone flat and the tread was coming apart.

Stuck in a middle lane, Sumner had to gingerly drive about a mile and a half before he could safely pull onto the shoulder.

“I don’t think I ran over anything, so the tire probably was just out of air. I decided then I really wanted tire pressure monitors on my next car,” said Sumner, who acknowledges that he doesn’t check his tires regularly with a gauge.

His next car, a 2003 Cadillac CTS, didn’t come with tire pressure monitors, or TPMs, so Sumner had an aftermarket system installed for about $300. “It took a lot of hunting on the Internet to find it, but I’m a gadgeteer, and this is the gadget I wanted on my car,” he said.

Congress mandated TPMs in response to the 2000-01 Firestone tire recall and nearly 300 deaths from rollover accidents in Ford Explorers, most with Firestone tires. TPMs are supposed to warn drivers of low air pressure, cited as a key factor in Firestone tire failures. Low pressure causes tires to run hotter and weakens the rubber, leading to blowouts or tread separation.

Automakers were to phase in TPMs on new vehicles beginning with the 2004 model year, but the mandate is on hold because of a court battle.

In the meantime, automakers are voluntarily installing TPMs on cars and light trucks, and suppliers such as SmarTire, which makes the system installed on Sumner’s Cadillac, are selling them through tire and parts stores.

There are two types of TPMs. Direct TPMs have pressure monitors on each wheel and warn when any tire is at least 25% low through a display on the dashboard. Direct TPMs also provide continuous pressure readings for each tire.

Indirect TPMs use anti-lock brake sensors to measure differences in rotational speed among the four wheels and warn when one tire is least 30% below the others. Indirect systems do not specify which tire is low.

With millions of TPMs on the road, drivers who have them generally praise them.

Rich Wegel, a retired engineer from Chicago, credits a direct TPM system with alerting him to a leaky tire on a used 1998 Corvette he bought.

A Goodyear store inspected the tire and found nothing wrong, yet Wegel continued to get low-pressure warnings. He returned to the Goodyear dealer and insisted it remove the tire from the wheel for a closer look, and that’s when it found a puncture that hadn’t been fully sealed.

After the tire was fixed, the low-pressure warnings ceased.

Wegel has replaced his Corvette with a CTS that doesn’t have TPMs.

“I wish it did. I think they’re excellent,” Wegel said, adding that his experience with the Corvette made him give his tires closer scrutiny.

Discussion boards on Edmunds.com, an automotive consumer site, included a Murano owner who said his Nissan dealer said it couldn’t rotate the tires because the TPMs work at only one location. Direct TPMs can be rotated but need to be recalibrated with a magnetic tool.

One befuddled owner on Edmunds said the direct TPM on his Infiniti FX45 was “really nice but the display is confusing because it does not label which reading belongs to which tire, which I find rather odd.”

Infiniti spokesman Bill Garlin said tire pressures were displayed “in a random order,” and if one was low, the driver was advised to check all four.

“But at least you know to check,” Garlin said.

John Rastetter, chief tire tester for TireRack, a mail order and Internet retailer based in South Bend, Ind., said pushing a button on the dashboard to check tire pressures could be misleading.

Tires that have been sitting in the sun for a few hours may read 5 pounds per square inch higher than normal, for example, and tires lose about 1 pound of pressure for each 10-degree drop in air temperature.

Car owners still need to check tire pressure with a gauge when the tires are cold and inspect the tires for nails, cuts and other damage, Rastetter said.