We want to remind our readers that all 2008 model-year vehicles are equipped with a tire-pressure monitoring system that illuminates an on-dash warning light when the pressure in one or more tires is low. Prior to 2008, tire monitor systems were not federally mandated.

However, AAA warns motorists that the TPMS should not replace manual monthly tire-pressure checks. The systems are calibrated not to trigger the warning light until a tire is at least 25 percent underinflated -- well below the pressure required for safe driving.

Two types of tire-pressure monitoring systems are in use -- direct and indirect sensing.

Direct-sensing systems are more accurate and use a sensor inside each tire to detect the pressure to within 1 pound per square inch. Still, even with direct-sensing systems, the warning light will not illuminate until a tire is at least 25 percent underinflated. Indirect-sensing systems do not monitor the actual pressure in each tire and are therefore less sensitive. Instead, they monitor the anti-lock brake system wheel speed sensors to identify when one tire is significantly underinflated compared to the others. Again, this system will not turn on the tire-pressure warning light until one tire is underinflated by at least 25 percent. And they will not detect low tire pressures at all if all four wheels deflate equally.

Driving on underinflated tires is unsafe, wastes fuel and increases tire wear, so AAA recommends motorists use a gauge to check tire pressure monthly. And, as colder weather rolls in, it's extra important as tires lose 1-2 pounds of pressure for every 10-degree drop in outside air temperature.

To learn more, visit the automotive page of AAA.com.

Write to Greg Zyla in care of King Features Weekly Service, P.O. Box 536475, Orlando, FL 32853-6475, or send an e-mail to gzyla@ptd.net. (c) 2007 King Features Synd., Inc.