Feeling Under Pressure?

If you plan to buy a new 2008 vehicle you will find it equipped with a new feature that could save gasoline and, more importantly, save lives. The National Highway Traffic and Safety Administration (NHTSA), enforcing the provision of the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act, has required any vehicle with a gross vehicle weight rating less than 10,000 pounds sold on or after September 1, 2007, must be equipped with a tire pressure monitoring system (TPMS). Many manufacturers are getting the jump on this regulation by installing the systems in the 2007 model year vehicles that are on sale right now.

Tire pressure monitoring systems are designed to alert the driver if the air pressure in one or more of their tires requires attention. What this means for consumers is that their vehicle will help them pay attention to their tires, which is a good thing. Improperly inflated tires not only reduce fuel economy and tire life, they can also compromise handling and safety, as we have found with the rash of single-vehicle tire-related crashes that have occurred in recent years.

"The most important thing that customers need to realize," Tom McInerney, technical service engineer for Ford Motor Company, "is that changes in outdoor temperature will increase or decrease tire pressure by approximately one psi (pounds per square inch). For example, if you park your car in a heated garage and it's minus 30 degrees outside, you'll need to add about seven pounds of air to the tires to compensate for the change in temperature."

While the TPMS's should enhance safety overall, they are no substitute for actually paying attention to your tires. The sensors are not required to illuminate a warning until tires are 15 percent below their certified inflation pressure -- the pressure recommended in the owner's manual and printed on the door-jamb label. That means possible under-inflation can affect handling and tire life before the sensing system registers a problem. Most tire experts recommend that drivers check the air pressure in all their tires, including the spare, at least once a month. McInerney believes tire pressure should always be checked when the tires are cold and suggests that customers should purchase a good digital or dial tire pressure gauge. He says the stick-type gauges are highly inaccurate.

Like many other vehicles new to the market this year, 2007 Ford and Lincoln Mercury vehicles are equipped with a direct sensing system. Each of the four road tires has a sensor mounted in the valve stem. This sensor reads the air pressure of each tire, transmitting data every minute once the vehicle reaches 20 mph and every hour when the vehicle is traveling at less than 20 mph or at rest. When a sensor detects that the tire is under-inflated, it sends a signal that illuminates a warning light on the vehicle's instrument panel. A solid light indicates that one or more tires need air. A flashing light indicates a system fault, requiring that the vehicle be brought into the dealer for service.

The warning light will reset itself and turn off once tire pressures are set to recommended specifications and the vehicle has been driven above 20 mph for at least two minutes.
McInerney reminds customers not to drive on a flat tire if at all possible as it could damage the tire pressure sensor, and damage to the sensor is not covered under warranty. Sensors can also be damaged if the repaired or replacement tire is improperly mounted.

Tire Pressure Monitoring Systems are required by the federal government and cannot be disabled by the dealer. And for those of you who covet "rims," you should know that aftermarket wheels and tires are not recommended as they may cause the system to malfunction.

Driving Today Contributing Editor Tom Ripley reports on the auto industry, vehicle safety and the human condition from his home in Villeperce, France.