Checking tire pressure and why it matters
Correct tire pressure is listed on a placard on the car, not on the tire sidewall. Here is how to find that number, check cold tires with a gauge and spot a slow leak.
The number that matters for tire pressure is printed on the car, not on the tire. The maximum pressure molded into the sidewall is the tire's limit, not the recommended inflation for your vehicle. The correct cold pressure is listed on a placard that gives the values for the front and rear axles, and keeping to it keeps handling predictable and wear even.
Underinflated tires build up heat more quickly, flex more, wear faster at the edges and respond more slowly to steering. Overinflated tires have a smaller contact patch, which reduces grip and makes the ride harsher, and they wear fastest in the center. Both extremes cost money and reduce the safety margin, which is why a few minutes with a gauge is well spent.
Finding the right number
The placard is usually on the edge of the driver's door or the door frame, and some cars list the same information in the glove box, on the inside of the fuel filler door or in the owner's manual. It shows the recommended cold pressure for the front and rear tires, often in both pounds per square inch and kilopascals. Use the values for the tire size fitted to your car, because a different size can call for a different pressure.
The placard also reflects the load the vehicle was designed to carry. Some cars list a different pressure for heavy loads or for a full set of passengers, so check the placard or the manual before setting pressures for a trip with heavy cargo or a trailer. Adding air above the placard value to compensate for weight is not a substitute for reading the correct figure, and the maximum printed on the sidewall is never the target.
The spare tire needs attention too. Compact temporary spares usually run at a much higher pressure than standard tires, and that value is stamped on the spare itself or listed in the manual. Check the spare occasionally, because a spare that has sat unused for years can lose pressure without anyone driving on it, and a flat spare is no help on the day you need it.
Checking cold tires with a gauge
Check tire pressure when the tires are cold, which means the car has been parked for several hours or has been driven no more than about a mile. Driving warms the air inside the tires and raises the reading by a noticeable amount, so a check taken after a long drive can make a correctly inflated tire look high. Remove the valve cap, press the gauge straight onto the valve stem so that no air hisses out, read the number and then replace the cap.
Use a gauge you trust. A pencil gauge, a dial gauge or a digital gauge will all do the job if they are accurate. Air machines at fuel stations can be less accurate than a personal gauge, so confirm the result with your own. Check all four tires plus the spare, and if you add air, recheck with the gauge afterward, since it is easy to overfill a tire by a few pounds per square inch.
Temperature changes the reading as well. Air pressure drops by roughly one pound per square inch for every ten degrees Fahrenheit that the air temperature falls, so a tire set correctly in warm weather can read low on a cold morning. Check the pressure again when the seasons change, and keep the tires at the placard value as conditions change.
Keep the valve caps on the stems when you are not checking, because they keep dirt and moisture out of the valve core. Check at the same point each time, use the same gauge, and write the readings down so that small changes stand out. The routine takes only a few minutes per visit and makes a developing problem much easier to spot.
Spotting a slow leak
A slow leak loses pressure gradually, often over days or weeks, and may not trigger a warning right away. Common causes include a nail or screw in the tread, a damaged valve stem, a bead that is not sealed to the rim after a tire change, or a loose valve core. If one tire needs air more often than the others, write down the date, add air to the placard value and recheck it in a few days to confirm the pattern.
To find a small leak, brush soapy water over the tread, sidewall and valve stem, then look for bubbles. Bubbles at the valve stem often point to a loose valve core or cap, which can be tightened. A puncture that has lost most of its pressure should be inspected by a tire shop, because the repair depends on where the damage is. Do not keep driving on a tire that is clearly low or flat. Underinflated tires can overheat and fail, and the tire pressure warning light is a late signal, since by the time it appears the tire is usually well below the placard value.
A simple routine catches most problems early. Check all four tires and the spare at the same time each month and before long trips, and record the readings in a phone note so that you can compare them over time. A tire that drops a few pounds between checks is worth watching, while a tire that loses a large amount between checks has a problem that needs a prompt visit to a tire shop.