Keeping your tires properly inflated is one of the easiest ways to help maintain good gas mileage and extend the life of your tires. Check your car's tire pressure at least once a month with a quality gauge, that measures pressure in pounds-per square inch (psi). Three types of tire pressure gauges are available at most auto parts stores: digital, dial and pen/stick types. The digital and dial designs tend to be more accurate and easier to read, although a good pen/stick gauge will do the job as well. Pressure gauges built into air hoses at gas stations are often abused and frequently inaccurate.
Recommended tire pressures are for cold tires. Therefore, tire pressure should be checked at ambient temperature before the vehicle has been driven. Checking tire pressure on a car that has warm tires can result in a pressure reading of up to 5 psi higher than the recommended pressure. The recommended inflation pressures for your car's tires can be found in the vehicle's owner's manual or on the tire information decal attached to the driver's door jamb. On older cars the decal may be in the glove box or inside the fuel filler door.