Tire Pressure Monitors Required by 2003

July 27 -- When was the last time you checked the air pressure in your car's tires? Chances are, not lately.

"We're constantly finding that tires are worn, improperly inflated, sometimes under-inflated, sometimes over-inflated," says mechanic David Schaffer.

And there is little doubt that improperly inflated tires are the source of many vehicular problems. For example, under-inflated Firestone tires are being blamed for many of the 203 deaths involving Ford Explorers last year.

Those tragic accidental deaths — and the subsequent industry-wide recall of 6.5 million Firestone tires — has prompted the National Highway Traffic Safety Administration to mandate that all vehicles made after 2003 to come with so-called tire pressure monitors.

The Way the Wheel Turns

There are two major kinds of tire pressure monitoring systems (TPMSs) currently available: indirect and direct.

An indirect TPMS, the cheaper and more common option, uses the wheel-speed sensors of the car's existing anti-lock brake system (ABS) to keep track of each wheel's rotation. An under-inflated tire has a smaller radius and therefore spins faster than the other properly inflated tires. When the indirect TPMS system detects such anomolies, it alerts the driver that they need to check their wheels.

Indirect TPMS systems are simple and highly affordable. In fact, NHTSA estimates that installation in a car with ABS would cost less than $13.

But indirect systems have some limitations. Since it relies on the rotational speed of the wheels, indirect monitoring systems may not be able to detect slight changes in tire pressure.What's more, they won't detect a problem at all if all four tires lose similar amounts of pressure.

A more accurate TPMS relies on actual pressure sensors installed in all four tires and can accurately measure the pressure in each tire. The readings are sent over radio waves to a wireless receiver mounted in the car and can warn drivers which specific tire needs attention.