If you're driving a 2008 or newer car, truck or SUV, it has a tire pressure monitoring system (TPMS). The feature became standard on all 2008 and newer models, thanks to the TREAD Act that Congress enacted in 2000 after rollover incidents involving the Ford Explorer and Firestone tires. Some 2006 and 2007 model-year vehicles also have TPMS.
The TPMS symbol is either a cross-section of a tire with an exclamation point in it or an overhead view of a car with all four tires exposed, according to the National Highway Traffic Safety Administration (NHTSA). Because of 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 below the manufacturer's recommended tire pressure. That's also well below the pressure required for safe driving, according to the American Automobile Association.
This situation exists 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 on a slightly underinflated tire while carrying a minivan full of high school football players or a pickup truck with a bed full of lumber, that tire could overheat and blow out.
Know the Light and Know What To Do
When the TPMS light illuminates, you should check the tire pressure on all tires. Only one may need air, but checking all of them is a good habit to have. If the light is flashing, there could be a malfunction with the TPMS system. It could also mean that you are using a spare tire and the vehicle cannot detect the sensor of the original wheel.
In theory, a TPMS is just one more feature that helps drivers understand the safety of their cars. But it's only effective if drivers can identify the light and are still vigilant about checking their tire pressures.
A study by Schrader International, a company that manufactures TPMS systems, found that 42 percent of drivers are unable to identify the low-tire-pressure warning light on the instrument cluster. Roughly the same percentage of those polled admitted to rarely checking the tire pressure.
Once the light has come on, some people might wait days to get around to filling their tires. The same Schrader study indicated that for those who do recognize the TMPS light, 21 percent said that when they stop to check the low tire, they only give it a visual inspection, rather than using a tire pressure gauge. Worse yet, roughly 10 percent admitted to ignoring the light altogether.
People who rely on the TPMS to warn them about low pressure are taking their chances. In addition to being a safety hazard, low tire pressure decreases fuel economy and causes tires to wear out more quickly. These are all reasons to be vigilant about checking tire pressure at least monthly, and to not rely on a TPMS to do the job.
Losing Control Before the TPMS Illuminates
A tire that's underinflated by just 5 psi can potentially fail. 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 will heat up and snap. It's especially important to be vigilant about tire pressure when the weather is hot and vehicle speeds are high.
Temperature is another factor that can affect tire pressure and trigger your TPMS light. When the weather is cold, the tire pressures will drop. When it warms up, the pressures will rise. This is something to keep in mind when the fall or summer comes along. For every 10 degree change in ambient temperature, you lose or gain about 1 psi, according to Tire Rack.
Where Do Automakers and the Government Stand?
So why didn't NHTSA require that the warning light illuminate sooner, before the pressure dips too far? "The TPMS regulations were meant to warn drivers that a tire failure is imminent, not to indicate unsafe handling might occur," according to the agency.
From the standpoint of the automakers, having a TPMS that activates at a lower threshold is problematic because it introduces the possibility of false warnings. 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
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 sensor 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, a direct TPMS measure a tire's actual pressure. They are usually 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. How the data is interpreted and displayed in the car depends on whether it is a low-line or high-line unit.
A high-line system has sensors mounted in each of the wheelwells and will display the individual pressure of each tire on the instrument cluster. Low-line systems are found on less expensive cars and will only prompt the low-pressure warning light. It is then up to the driver to figure out which tire is low. Much more useful to drivers, clearly, are systems that actually display the pressure of each tire.
How can you tell whether you have an indirect or direct TPMS system? There's no easy way to tell without looking into the tire, but a general rule is that 2008 and newer vehicles all have direct systems. The only way for automakers to comply with the NHTSA requirements was to go with a direct system.
You Are the Best Tire Pressure Monitoring System
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 should be it: It's your responsibility as a driver to check your tire pressures monthly, or at least have them checked by someone else.
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 is not on the tire.
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