Check Air Pressure in Tires

Incorrect pressure can cause poor mileage, uneven tire wear, or a tire blow-out. To prevent these events from happening it is important to maintain proper tire pressure. Check your tire pressure every time you fill your car with gas to ensure that you're getting the most out of your tires - and that your tires are being treated well.

Steps

Checking Air Pressure in Tires

  1. Look in the owners' manual or on the inside of the driver's side door for the standard cold tire inflation pressure. This number is the lowest PSI one would inflate the tires to and is suggested by the car's manufacturer. Read below for reasons inflation may be placed higher.
    • For most sedans, minivans, and even mini pickups, manufacturers generally recommend a PSI (pounds per square inch) in the range of 27 to 32, but can reach all the way up to 40.[1]
    • For larger vehicles that need to carry a larger burden, such as trucks and SUVs, the PSI is generally 4 to 8 PSI greater than it would be in smaller cars, perhaps around 45.
    • Also note that the front and back tires may need different pressures, according to the manufacturer.
  2. Unscrew the valve stem cap from the valve stem on the tire. The valve stem is a black or silver pencil-sized extension near the hubcap, about 1" (2-3 cm) long.
  3. Press the air pressure gauge evenly onto the valve stem and record the reading given. If there is a hissing sound, the gauge is not tight or even enough for an accurate reading. The angle of the gauge may need to be adjusted.
    • If you are using a digital model gauge, you may or may not need to press a button in order for the gauge to read the air pressure. If you are using a traditional gauge, the metered stick should give you a reading automatically.
  4. Replace valve stem cap. The cap does not hold air in, but it keeps dirt and moisture away from the valve mechanism in the valve stem, which does hold air in.
    • Note that if the reading is the same as the manuals' specifications, you are done after checking all other tires for the same pressure. If inadequate pressure is in the tires then fill air in the tires. Make sure you put in the correct amount.

Performance Considerations

  1. Know that the manufacturer's PSI recommendations don't translate into optimum tire performance. For all-around driving, the manufacturer's specifications are probably ideal, but adding a couple extra pounds of pressure into your tires may mean better fuel efficiency. Overall, adding a few pounds of PSI to your tires may make your ride a bit bumpier and less pleasant, so use with tact!
    • An increase in PSI can also result in uneven tire wear, longer required braking distances, and reduced handling. Make sure you don't over-inflate your tires.
  2. Understand the myth about the max press value in the owner's manual or driver's side door. One popular misconception is that the max pressure suggested by the manufacturer is all the pressure the tire can handle before it pops or malfunctions. In truth, the max pressure is the pressure at which the tires will carry the maximum amount of weight. [2]
    • As soon as you inflate the tires past the max pressure limit, be prepared for the possibility of malfunction. If your tires are bearing heavier air pressure, a pothole at high speeds could spell disaster.
  3. Add a few extra pounds of pressure to the back tires if you're carrying extra cargo, carpooling, etc. If you happen to be carrying a heavier load in your car, especially if you are traveling for longer distances, don't be afraid to add a few pounds of PSI to your rear tires to offset the added weight. When the weight is unloaded, release the pressure from the rear tires back to its standard specifications.
  4. Check the air pressure of your tires as the seasons change. Cold weather will reduce the air pressure, while warm weather will increase the air pressure. It's therefore very important to check tire pressure when the seasons change.
  5. Never rely on the eyeball method to gauge air pressure in tires. Don't get lazy. It's very difficult to tell the difference between a tire with 10 PSI and 20 PSI. Plus, tires normally exhibit a bit of a bulge on the sidewall of radial tires. If you inflate the tires until the bulge is gone, you run the risk of seriously over-inflating your tires, past the point at which they get a performance boost.



Tips

  • Tires cannot be eye-balled for pressure, particularly modern radials. Always use an accurate gauge.
  • Sunlight heats up tires even if they're not driven. For more even readings take note that not one side of the car has sun shining on it.
  • Take action if you see the tire pressure monitor system (TPMS) light come on in your vehicle. The TPMS indicator is a yellow symbol on the dash that will light up if one or more tires is under-inflated.
  • The PSI listed on the sidewall of the tire is the max cold pressure for the tire carrying the highest (weight) load the tire supports.
  • If the car has to be driven to add air note the pressure before driving away. Then add the difference above when the reading is now. For instance if you wish to inflate your tires to 35psi and they're reading 30psi cold. The tires are 5psi under-inflated so when you add air after driving and they now read 33psi adjust them to be 38psi. They should then read 35psi when cold.
  • Note that tires have a speed rating in the form of a letter. For example a zr rating has a maximum speed of 149 mph. You can go faster than that for brief periods of time but the tire may fail. The speed rating is only good for new tires, If the tire has 20,000 miles on it the max speed is probably less due to general wear and tear. Once a tire has been repaired the speed rating is void.
  • Increase the cold tire pressure if the car will be carrying a heavy load or driven at high speeds (over 75 mph) for long times.

Warnings

  • An over-inflated tire (filled above the max cold pressure on the sidewall) makes for a harsher ride and makes it more prone to damage if you hit pot-holes or other objects in the road.
  • An under-inflated tire causes more sidewall flexing that increases stopping distance, lowers fuel economy and shortens the life of a tire. In rare cases the tire can blowout because of excessive heat from too much sidewall flexing and can even roll off the wheel in emergency maneuvers. Under-inflated tires will wear down the sidewall by making contact with the rim and the road.
  • Do not rely solely on the air pump gauge at the gas station. These pumps take a lot of abuse (slammed against concrete, ran over, etc.) and may not be calibrated properly. Always base your final readings using your gauge for best accuracy.

Things You'll Need

  • Tire pressure gauge of good quality. Do not rely on 'pencil' type tire gauges. Professional quality tools always pay for themselves over time; buy a professional quality tire gauge.
  • Air compressor or pump with correct fitting (bicycles can have one of two types: Presta or Schrader - Schrader is the type used by cars and assumed in the description above)...

Related Articles

Sources and Citations

  1. ↑ http://www.dmv.org/how-to-guides/check-tire-pressure.php
  2. ↑ http://www.popularmechanics.com/cars/how-to/repair/6-common-tire-myths-debunked-10031440
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