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Tire maintenance for motor vehicles is based on several factors. The chief reason for tire replacement is friction from moving contact with road surfaces, causing the tread on the outer perimeter of tires to eventually wear away. When the tread depth becomes too shallow (less than 0.125in./3.2mm), the tire is worn out and should be replaced. The same wheels can usually be used throughout the lifetime of the car. Other problems encountered in tire maintenance include:
- Uneven or accelerated tire wear: can be caused by under-inflation, overloading or poor wheel alignment.
- Increased tread wear on only one side of a tire: often a sign of poor wheel alignment.
- Tread worn away completely: especially when the wear on the outer rubber exposes the reinforcing threads within, the tire is said to be bald and must 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 wheel rim, to be used in the event of a flat tire or blowout. Spare tires (sometimes called "doughnuts") for modern cars are smaller than regular tires (to save trunk space, weight and cost) and should not be used to drive very far before replacement with a full-size tire. A few vehicle models use conventional size spare tires. Jacks and wrenches for emergency replacement of a flat tire with a spare are included with a new car. Not included, but available separately, are hand or foot pumps for filling a tire with air by the vehicle owner. Cans of pressurized air can sometimes be bought separately for convenient emergency refill of a tire.
Some 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 tire change or calling for roadside assistance.
Interestingly, tires actually have more traction when they are bald, because there is more surface area making contact with the road. The reason regular tires have treads is to avoid hydroplaning when the surface of the road is wet. Stock cars driven on professionally maintained NASCAR tracks use tires without treads, and with a thicker layer of rubber.
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.
Inflation (adding air)
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. Accurate readings can only be obtained when the tires are 'cold' - that is at least three hours after the vehicle has been driven or driven less than 1/2 mile since cold - tire pressures will not then be higher because of operating heat. The recommended inflation pressure is found in the owner's manual and on the vehicle's Tire Placard.  Because of slow air leaks, changes in the weather and ambient temperature or other conditions, tire pressure will occasionally have to be corrected via the valve stem with compressed air which is often available at service stations.
Under-inflation of tires can cause premature tire wear and carries an increased risk of explosive failure (blowout) especially after prolonged high speed operation at high temperatures. Many vehicles have tire pressure monitoring systems; older cars are usually equipped with indirect monitoring systems while later cars are typically equipped with direct tire pressure monitoring systems.