Tire maintenance

Friction from moving contact with the road causes the tread on the outer perimeter of the tire to eventually wear away. When the tire tread becomes too shallow, the tire is worn out and should be replaced. The same wheels can usually be used throughout the lifetime of the car. Uneven or accelerated tire wear can be caused by under-inflation, overload or bad wheel alignment. Greater wear on a tire facing the outside or the inside of a car is often a sign of bad wheel alignment. When the tread is worn away completely, especially when the wear on the outer rubber exposes the reinforcing threads within, the tire is said to be "bald" and should be replaced as soon as possible. Sometimes tires with worn tread are "recapped", i. e. a new layer of rubber with grooves is bonded onto the outer perimeter of a worn tire. Since this bonding may occasionally come loose, new tires are considered superior to recapped ones.

Sometimes a pneumatic tire gets a hole or a leak through which the air inside leaks out resulting in a flat tire, a condition which must be fixed before the car can be driven safely. See Flat tire for more information.

Vehicles typically carry a spare tire, already mounted on a rim, to be used in the event of flat tire or blowout. Many spare tires (sometimes called "doughnuts") for modern cars are smaller than normal tires (to save on trunk space, gas mileage, weight and cost) and should not be driven very far before replacement with a full-size tire. A few modern vehicle models may use conventional spare tires. Jacks and for emergency replacement of a flat tire with a spare tire are included with a new car. Not included, but sometimes available separately, are hand or foot pumps for filling a tire with air by the vehicle owner. Cans of pressurized "gas" can sometimes be bought separately for convenient emergency refill of a tire.

Some modern cars and trucks are equipped with run flat tires that may be driven with a puncture over a distance of 80 km to 100 km. This eliminates the need for an immediate stop, and the associated expensive tow service or tire change.

Front tires, especially on front wheel drive vehicles, have a tendency to wear out more quickly than rear tires. Routine maintenance including tire rotation (exchanging the front and rear tires with each other) is often done periodically to facilitate uniform tire wear. There are simple hand-held tire-pressure gauges which can be temporarily attached to the valve stem to check a tire's interior air pressure. This measurement of tire inflation pressure should be made at least once a month. The [http://www.nhtsa.dot.gov/cars/testing/ncap/Tires/pages/PSI.htm proper] inflation pressure is located in the owner's manual and on the Tire Placard. Because of slow leaks or changes in weather or other conditions tire pressure may occasionally have to be corrected, usually via the valve stem with compressed air which is often available at service stations.

Under-inflation of tires can also cause premature tire wear. Many modern vehicles have tire pressure monitoring systems; older cars typically equipped with indirect monitoring systems while later cars are typically equipped with direct monitoring systems. See Tire Pressure Monitoring System for more information.