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The safety and gas mileage benefits you'll get by maintaining your car’s tire pressures is well worth the small effort.

Braking, cornering, stability, and comfort all suffer if tire pressures aren’t correct. Under-inflated tires run hotter, wear faster, and are more subject to tread separation, which can lead to sudden failure and accidents. Over-inflation causes tires to bounce instead of absorbing some shocks, and reduces traction.

On the mileage front, under-inflated tires mean too much of the tire’s tread contacts the road. That causes unnecessary friction and lower gas mileage. The General Accounting Office estimates that if we all did our part, U.S. drivers would burn 1.2 billion fewer gallons of gasoline per year, a savings of one percent. The Environmental Protection Agency and Department of Energy estimate that if your tires are underinflated by as little as five or 10 pounds per square inch (PSI), you could gain as much as 3.3 percent in mileage by simply bringing your tires up to spec. And there’s a good chance you need to: the National Highway Transportation Safety Administration found in 2007 that roughly one in four cars and light trucks had tires were at least 25 percent under-inflated.

checking tire pressureAnytime you get your car serviced, make sure the shop checks and adjusts tire pressure.

Between service appointments, check pressures yourself—monthly is a good rule of thumb. Tires lose pressure through normal use—about one PSI a month. Seasonal temperature differences greatly affect pressure; the pressure goes up or down about one PSI for every 10-degree change in air temperature. Regular checks will also alert you to leaks and other problems.

You can’t tell if your tires are inflated correctly by just looking. A radial tire that’s 30 percent under-inflated can look identical to a properly inflated tire. Get a gauge you can keep handy in your glove compartment. Avoid the gauges that are built in to gas station compressors; they’re usually beat-up and inaccurate. The pencil-style gauges—the kind with the indexed slide that pops out—are fine. But you might consider spending a little extra for a dial-type gauge or a digital gauge. They’re easier to read, and typically more accurate. Some other tips:

  • Use your car manufacturer’s recommendations, which you can find on the driver’s door near the latch mechanism (open the door to see it), the gas filler door, the glove compartment door, or under the hood. If you don’t find them, check your owner’s manual. Do not use the numbers on the tires. These are minimums and maximums for the tire, not optimal operating pressures for your car.
  • Check tires when they are “cold”—when the car has been sitting overnight, or hasn’t been driven for a few hours. Driving a few blocks to the gas station—a good guideline is less than a mile—is fine. But more than that will yield an inaccurately high reading because the air inside the tire will have been heated. If you have a portable compressor that runs off your car’s battery power, check and adjust pressures in the morning before you leave home.
  • At the gas station, remove all the valve caps first to save time while using the station’s compressor. Have your gauge handy. Add pressure—it’s best to add a little too much, and then release extra pressure, check again, and repeat until you get the right reading.
  • Check all four tires and your spare—even if you have one of those little, space-saver spares. Space-savers require a different pressure than the other tires, but unlike regular tires, the number on the spare itself is what you should go by.
  • Finally, screw on the valve caps tightly; they help reduce leakage, and should the tire valve fail, they’ll help maintain pressure and avoid a sudden, dangerous deflation.

All late-model cars come equipped with a basic tire pressure monitoring system that warns drivers when tire pressure drops more than 25 percent below recommended levels. If your car isn’t equipped with one of these systems, you can buy after-market sensors that screw onto the valve stems and will provide a low-pressure warning. The warning-type devices are good safety nets, but you’ll still want to check tire pressures regularly. A typical recommended tire pressure is 32 PSI. Under-inflation by, say, six PSI is common, and substantially increases tread wear and decreases mileage—but is still short of the 25 percent warning threshold.