WASHINGTON - A light on motorists' instrument panels will soon warn them when a tire is underinflated.

The safety regulation, issued by the government Thursday, has its roots in the Firestone tire recall of 2000. It requires new passenger cars to have tire pressure monitoring systems in place by the 2008 model year.

Automakers most likely will attach tiny sensors to each wheel that will signal if a tire falls 25 percent below the recommended inflation pressure. If any one of the four tires is underinflated, the sensor sets off a warning light.

Automakers will begin using the technology in September. The National Highway Traffic Safety Administration estimates the upgrade will cost manufacturers between $48.44 and $69.89 per vehicle.

The government said underinflated tires affect a vehicle's fuel economy and can increase stopping distances, contribute to the likelihood of tire failure and lead to skidding on wet surfaces. One NHTSA survey found about 30 percent of cars and light trucks have at least one tire underinflated by 8 pounds per square inch or more.

All new four-wheel vehicles weighing 10,000 pounds or less must be equipped with the systems by the 2008 model year, the government said. The regulation affects passenger cars, sport utility vehicles, pickup trucks and minivans.

NHTSA estimates that 120 lives a year will be saved when all new vehicles are equipped with the systems.

The regulation was proposed last September. Tire manufacturers have questioned whether the warning system would signal low pressure early enough. Automakers have raised concerns that motorists may ignore the lights if they appear too frequently.

Donald Shea, president and chief executive of the Rubber Manufacturers Association, the trade group that represents tire makers, said, "Unfortunately, this regulation may give motorists a false sense of security that their tires are properly inflated when they may be significantly underinflated."

Eron Shosteck, a spokesman for The Alliance of Automobile Manufacturers, which represents nine automakers, said about 18 percent of their vehicles already have the technology. It first appeared in the 1997 Chevrolet Corvette and is currently used in some luxury vehicles.

"We're gratified that there's a final rule which allows us to continue the implementation of the technology as we've been doing," Shosteck said. Automakers estimate the upgrade will cost an average of $115 per vehicle.

Congress, aiming to prevent SUV rollovers after the massive Firestone tire recall began in August 2000, sought the warning devices in the Transportation Recall Enhancement, Accountability and Documentation Act.