Have you checked your tires lately?

The National Highway Traffic Safety Administration says at any one time, more than one-quarter of vehicles have at least one tire that's seriously under-inflated.

As it marks National Tire Safety Week, NHTSA is urging car owners to get into the habit of monitoring their tire pressure at least once a week. Officials say low tire pressure is a factor in 660 deaths and 33,000 injuries on the road each year.

NHTSA recently issued a new regulation requiring cars to be equipped with a dashboard warning system to tell drivers if they have a tire that's too low. But it won't take effect until the 2008 model year.

Administrator Jeffrey Runge warned people that they don't want to pump up your tires to that number that's printed on their sidewalls. That's usually the maximum inflated pressure.

He says the correct pressure number can be found on the door jamb inside the door on the driver's side. It's called "placard pressure."

The agency also dedicates part of its Web site at SaferCar.gov to tire safety tips.

It points out that modern, radial tires make it very hard to visually tell if a tire is low, and that you need to buy a guage to make sure the pressure is correct. You should also take readings on tires that have not been driven for three hours.