When the seasons are changing, as they are now, it's the time to make especially certain your automobile's tires are properly inflated.

Though the tire pressure should be checked regularly -- as often as once a month, say tire experts -- it is when the weather turns cold that your tires lose air more rapidly.

The loss, say Firestone tires, amounts to about a pound of pressure for every 10-degree drop in temperature.

Firestone representatives recently demonstrated the firm's self-sealing radial 721 line, which they say provides "safety and convenience" for motorists who might otherwise have to change a flat on a busy highway in bad weather.Uniroyal also markets its Royal Seal, and Goodyear has one in the works.

To test the tire's air-holding capacity, tire engineer Tom Bell and a reporter each drove two 2-inch-long nails into the tread of the 721, pulled out one of them and then rode on the tire for 45 minutes without discernible pressure loss.

In tire care, "underinflation," says the Tire Industry Safety Council, which represents the nation's tire manufacturers, "is a tire's worst enemy." It can cost you in more rapid wear of the tread and reduced fuel efficiency.

And, says BF Goodrich, "Unequal pressure among the four tires can lead to uneven braking and poor handling."

The council estimates that someone who drives 10,000 miles a year "can gain approximately 300 extra miles on the same amount of gasoline by checking and adjusting the air pressure in your tires on a monthly basis.

"Underinflated, soft tires make the engine work harder to overcome increased rolling resistance. The more work your engine does, the more gas it uses."

A tire is considered "seriously underinflated," says the council, "when it is four or more pounds below the recommended air pressure."

While most drivers check their tire pressure at a service station, tire experts say that is wrong "unless you live less than a mile away."

Instead, they say, check the pressure at home with your own gauge while the tires are "cold" -- that is, when they haven't been driven for three hours.

If, for example, your "cold" tire is six pounds under pressure, add six pounds to the reading you get at the service station, even if it means going above the tire's recommended pressure.

A common misconception, says one manufacturer, is that lower air pressure improves traction. "Wrong. Lower air pressure does not improve traction."

For winter driving, the American Automobile Association recommends snow tires. But, it says, "chains provide the best starting and stopping performance in severe snow and on icy surfaces."

In areas of light to moderate snowfall, it suggests, consider a set of all-season radials. Remember, though, that "radical tires are not snow tires unless they have a snow tread configuration and are marked 'M&S' (Mud and Snow)."

The AAA offers these safety tips for winter driving:

Prepare for the cold by getting an engine tune-up. "Winter conditions -- wet, cold and icy weather -- present the greatest difficulty to your engine's operating efficiency." Check especially the battery, lights, ignition system.

Have your brakes checked. "To avoid skids on slippery surfaces, you need equal braking on all four wheels."

Keep the gas tank at least half full to avoid gas-line freeze-up.

Never warm up a vehicle in an enclosed area.

Beware of carbon-monoxide poisoning if your car is stuck in snow and the engine and heater are running. "Open a window slightly and clear snow away from the exhaust pipe."

Carry a "winter driving kit" that includes: chains, small shovel, extra clothing, traction mats, ice scraper, booster cables, warning devices; small bag of abrasive material such as sand, salt or kitty litter; snow brush, flashlight, cloth or roll of paper towels and blanket.

Clean snow from the entire car -- hood, roof, trunk and lights in addition to all the windows.

When starting, turn on the heater for a minute or two before using the defroster to prevent windshield fogging.

Drive "in the right frame of mind" -- always at a speed that matches the prevailing visibility, traffic and road conditions."