Tire Pressure Monitors? Can You Rely on Them?
Drivers Must Still Be Vigilant, Even With TPMS
By Mac Demere, Contributor edmunds.com

Here’s a quiz: What might it mean when your car’s tire-pressure monitoring system (TPMS) warning light is not illuminated?

a) Your tires may have plenty of pressure for all situations;
b) Your tire pressures might be so low that they may overheat and blow out;
c) Your tire pressures might be so low that the tires have little traction for wet roads or accident avoidance;
d) All of the above.

The answer is “d.” And “D” is the best grade some tire safety experts, consumer groups and drivers may give tire-pressure monitoring systems. Why? Because TPMS will warn you only when a tire is severely — perhaps dangerously — underinflated.

TPMS: Mandated by the Federal Government
If you’re driving a car, truck or SUV built in the past few years, there’s a good chance that it has a TPMS. Starting with all 2008 models, in fact, it’s a required feature. In response to the rollover incidents involving the Ford Explorer and Firestone tires, Congress enacted the TREAD Act in 2000. Part of this act got the process moving for having a TPMS in every vehicle.

An illuminated tire-pressure warning light symbol looks like the cross-section of a tire with an exclamation point in it. But due to a variety of considerations from tire companies and automakers, a TPMS warning light isn’t required to come on until a tire is 25 percent underinflated.

“[This is] well below the pressure required for safe driving,” says the American Automobile Association. This is partially because the recommended pressure for some vehicles is barely adequate to carry the vehicle’s maximum load, according to the Rubber Manufacturers’ Association (RMA). This means if you’re driving a minivan full of high school football players or a pickup with a bed full of damp mulch on a slightly underinflated tire, it could overheat and blow out.

Only as Good as the Driver
In theory, a TPMS is just one more feature that helps a driver understand the safety of his or her car. But it’s effective only if drivers are still vigilant about checking their car’s tire pressures.

People who rely on the TPMS to warn them about low pressure are taking their chances. A worrisome survey conducted by the RMA revealed that 40 percent of motorists say they would never check their tire pressure unless the TPMS light came on.

And once the light does come on, of course, some people might wait days to get around to filling their tires. In addition to being a safety hazard, low tire pressure decreases fuel economy and causes tires to wear out more quickly — all reasons to be vigilant.

Losing Control Before the TPMS Illuminates
From personal experience on the racetrack and test track, I know how poorly a car handles in emergency situations with a tire underinflated by even a small amount.

But everyday drivers are also at risk. During driving demonstrations, I’ve ridden with hundreds of non-professional drivers in cars with low air pressure. They drove around a wet-handling course in two identical cars: one with proper pressure and the other with rear tire pressure intentionally set 23 percent low.

When the rear tire pressure was low, many drivers lost control and spun out before they had completed a single lap.

If you reversed the situation — properly inflate rear tires but reduce pressure in the front tires — the car won’t respond appropriately when you turn the steering wheel. It will just plow straight ahead.

The accompanying photos, taken by Michelin engineers, explain much of what’s happening. A vehicle moving at 60 mph passed over a glass plate covered by 5mm of green-colored water. When inflated to the recommended 35 psi, the tire kept much of its tread on the surface. When pressure was lowered to 30 psi, less of the tire stayed in contact with the surface. When pressure was dropped to 25 psi, almost the entire tire literally floated on top of the water.

The accompanying infrared photos show that underinflating a tire just 5 psi can potentially cause a tire failure. An underinflated tire flexes more than a properly inflated tire, and that creates heat. Excessive heat can break down components and chemical bonds inside a tire: It’s much like bending a wire coat hanger: Bend it far enough and long enough, and it’ll heat up and snap. This is especially important when the weather is hot and speeds are high.

Where Do Automakers and the Government Stand?
So why didn’t the National Highway Traffic Safety Administration (NHTSA) require that the warning light illuminate sooner, before the pressure dips too far? According to NHTSA spokesman Eric Bolton, “The TPMS regulations were meant to warn drivers that a tire failure is imminent, not to indicate unsafe handling might occur.”

From the standpoint of the automakers, having a TPMS that activates at a lower threshold is problematic from a false warning perspective. Changes in temperature can have a dramatic effect on tire pressure. The concern is that frequent tire pressure warnings would cause drivers living in places with extreme temperature fluctuations to ignore the systems entirely.

Indirect Vs. Direct TPMS
Much more useful to drivers are the type of systems that actually display the pressure of each tire. There are two types of TPMS: indirect and direct. The lower-cost, indirect TPMS doesn’t actually monitor air pressure. Rather, indirect systems use the antilock braking system’s wheel-speed sensors to detect that one tire is rotating faster than its mates. (An underinflated tire has a smaller circumference so it has to roll faster to keep up.) Thus, the margin of error of indirect systems is large.

Meanwhile, direct TPMS measure a tire’s actual pressure. Expensive versions are accurate to within 1 psi. Current direct systems use a gauge mounted to the wheel or tire valve. This gauge sends a signal to the car’s computer. When you see the warning light from a direct system, trust it and immediately check your tire pressures.

Until recently, if a moderately priced car had TPMS, it was likely an indirect system. Only super-high-performance cars and those equipped with run-flat tires had the more expensive direct systems. In order to meet the full requirements of NHTSA’s TPMS standard, however, almost all new cars have direct systems. With direct TPMS, an automaker can also decide whether to display the actual pressures for each tire via a multifunction display or just rely on the warning light.

Rely on Yourself
On new cars, the automaker’s recommended pressure is on a placard on the driver’s doorjamb. On older cars it can be on the trunk lid, fuel door, glovebox, center console lid, passenger’s doorjamb or in your owner’s manual. It’s not on the tire.

Rely on TPMS to warn you only of a puncture or an active air leak. If you take away only one thing from this article, this is it: It’s your responsibility as a driver to check your tire pressures monthly, or at least to have them checked by someone else.

Mac Demere is a vehicle tester and race driver who competed in the NASCAR Southwest Tour and Daytona 24 Hours.