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 and uneven tire wear, excess fuel usage, and carries an increased risk of explosive failure (blowout) especially after prolonged high speed operation at high temperatures. Overall, these factors decrease the life of a tire by up to 20%. 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.