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A tire gauge measures the air pressure in any pneumatic (air-filled) wheel or tire. The results are usually read in PSI, Pounds (of air) per Square Inch. Automobile tires have recommended PSI levels imprinted on the tire itself and included on a chart inside the driver's side door panel. After a tire gauge is read, the PSI number is compared to the tire manufacturers' recommended PSI and compressed air may be added until that ideal pressure has been reached. Underinflated tires can cause poor fuel efficiency and an increased chance of tire failure. Overinflated tires can lead to dangerous blowouts and a noticeable loss of handling.
A commercial tire gauge comes in different forms. One of the most popular types resembles a ballpoint pen, complete with a clip for shirt pockets. On one end of the tire gauge there is a rounded tip with a small opening and a post in the center. This tip fits snugly over the inflation valve of a standard tire. As the user depresses the tire gauge over the valve, some air might escape from the tire. This shouldn't affect the actual reading much, but a good airtight seal is important.
On the other end of the pen-style tire gauge is a square opening which houses a plastic tube. On the side of this tube are calibration marks which denote pounds of air pressure. The air escaping from the tire valve forces the plastic tube out of the housing at a predetermined rate. Once the tube has stopped moving, the user can look at the last visible calibration mark to determine the tire's current PSI level. If the gauge reads 25 PSI and the recommendation is 35 PSI, then ten 'pounds' of air must be added to the tire. An air compressor with an appropriate tire valve fitting can add additional pounds until the tire gauge reads 35 PSI. Overinflated tires can be adjusted by manually depressing the tire's valve stem until enough air has been released.
Another type of tire gauge is attached to a commercial air compressor at service stations. Customers can check each tire's PSI readings for free by connecting the compressor hose to each valve stem. A metal tube performs the same measurements as the plastic version, with a spring to return it to the housing between readings. If a tire is found to be low in pressure, the customer can activate the coin-operated compressor and add more air. The attached tire gauge can be used at any time to measure the process.
Keeping tires properly inflated is something every driver can learn to do. It is especially important to check your tires before long trips and during extremely hot weather. Periodically check your spare tire's air pressure and any other vehicle which uses pneumatic tires, such as a riding lawnmower or trailer.