If you drive a '06 or '07 car, you might have noticed an unusual new warning light on the instrument panel. It looks like a stylized letter "U" cupping an exclamation point. Actually, the U represents a tire in cross-section and the exclamation point calls for your attention, because one of your tires has become dangerously deflated. On some cars, the warning light shows a schematic of a vehicle as seen from above; the low-pressure tire glows with a solid light while the other three remain in outline.

The warning light is part of the new tire pressure monitoring system required by federal law. All vehicles must have it by next year, but automakers have been phasing it in for a while. Currently, nearly half of all '06 models and about 70 percent of '07 models have some kind of system to monitor tire pressure.

Many experts offer a warning about the new warning systems, however. No matter how sophisticated, the monitoring devices absolutely, positively do not excuse you from checking your tire pressure manually at least once a month.

The problem lies in the pressure level that triggers the warning light. Under government regulations, the warning light will go on only when the pressure in one tire drops 25 percent below its recommended level. For example, for a tire with a recommended pressure of 32 psi, a drop to 24 psi or lower will trigger the light. That's simply too low, say the experts.

"Unfortunately, this regulation may give motorists a false sense of security that their tires are properly inflated when they may be significantly underinflated," says Donald B. Shea of the Rubber Manufacturers Association (RMA), a national group representing tire makers. Low pressure affects safety, because it causes tires to run hotter, resulting in damage and possible failure. It affects the vehicle's handling ability, too, especially on wet roads. And it affects fuel economy. For every 3 psi below the recommended pressure, your gas mileage drops 1 percent.

You can find the recommended tire pressure for your car on the tire inflation sticker, located on the edge of the driver's door or on the doorjamb.

"Tire pressure monitors are not a replacement for checking the tire pressure once a month with a tire gauge," emphasizes RMA's Shea. "Regular tire maintenance will become even more important now."