Having your car’s tires inflated to the right pressures can help with fuel economy, handling and tire life. So, if you’ve noticed that your car is sitting a bit lower to the ground or that a tire looks a little flatter, it may be time to add some air. But how do you know what the proper tire pressure is for your vehicle?
Related: What Does TPMS Mean?
Finding Your Car’s Recommended Tire Pressures
Nearly every vehicle lists the manufacturer-recommended tire pressures on a sticker affixed to the driver-side door jamb, which is the body pillar the door locks into. It could also be on the rear edge of the door itself, in the glove box or on the inside of the fuel-filler door. You can also find it in the owner’s manual.
Although this may seem like a clear, simple answer, there are a few things to keep in mind:
- The recommended tire pressure is a cold inflation pressure, which means the pressure check should be done after the vehicle has been sitting in the shade for a while. This can be trickier than you might think. Things that can throw off the reading include the car being in a garage that’s warmer (or colder) than the outside temperature, having the sun hitting one or two of the tires, or having driven on them for more than a couple of miles. (See the section on how temperature affects tire pressures below.)
- Some vehicles have different recommended inflation pressures for the front and rear tires, and sometimes a higher pressure (usually for the rear) will be listed for if you’re carrying a heavy load.
- If your car has a spare tire (some newer ones just have a sealing kit and a pump), don’t forget to check the pressure in that, as well, as air can leak out over time. (Note that some are small “doughnut” temporary spares that run much higher pressures than the regular tires.) Unfortunately, this isn’t always easy. Spare tires mounted under the rear of the vehicle are particularly troublesome in this regard, as they may need to be removed to check the pressure.
- The pressures listed are for the tire size that originally came on the vehicle, which is also listed on the sticker. If different-sized wheels and tires have been installed, the listed pressures won’t necessarily be optimal.
While the sidewall of the tire usually lists a pressure, that’s a maximum safe inflation pressure, not what’s recommended for its use on a particular vehicle.
How Temperature Affects Tire Pressure
A general rule of thumb often quoted is that tire pressure fluctuates by 1 pound per square inch for every change in temperature of 10 degrees, as air in the tire expands when it gets hotter (raising the pressure) and contracts when it gets colder (lowering the pressure). Though the rule is easy to remember, it isn’t really accurate for all tires and can be closer to 2% per 10 degrees.
This mostly affects you when the temperature drops in the late fall or winter if the tires were last checked and inflated in the heat of summer. In that case, a tire could easily lose 7-10 psi between June and January, and that could mean trouble. Low tire pressures can result in poor handling characteristics, particularly in emergency maneuvers; increased risk of a blowout; and premature tire wear.
Drivers of modern cars often get this called to their attention when the tire pressure monitoring system warning light, which typically looks like a “U” with an exclamation point in the center, illuminates on the instrument panel. The light usually comes on when the tires are determined to be 25% below their recommended pressures. This can also work in reverse: Tires properly inflated in the winter can be running too high a pressure as the weather warms; thus, some air should be let out to drop them to proper pressure — though running a slightly higher pressure than recommended isn’t as bad as running too low a pressure.
Driving the car will heat up the tires due to friction with the road, and it takes some time for them to cool down. Sun shining on the tires can also heat them up. In either case, the added heat increases the air pressure inside the tire, which is why it’s best to check pressures early in the morning after the car has been sitting overnight.
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Checking the Pressure
In some cars, the TPMS lets you go into a separate screen that reads out individual tire pressures, which is a real convenience. But if your car doesn’t have that feature, you’ll likely need to rely on a gauge.
While a regular push-on tire gauge is the most common and least expensive option, you can often have trouble getting it to seat properly. In some cases, that can not only result in an inaccurate reading, but it may also allow a fair amount of air to escape, which lowers the tire pressure.
Thus, it may be worthwhile getting a gauge that screws onto the tire valve, creating a more reliable seal. These gauges cost more — typically about $10 to $20 — but they usually include a large, easy-to-read dial along with a push-in button that allows you to easily release some pressure if it’s too high.
How Often Should I Check My Tire Pressures?
The common rule is to check your tire pressures once a month, but who wants to do that? Unfortunately, even that may not be enough. It’s ultimately your call on how frequently you check them, but a visual check (comparing the ground contact patch on the front tire to the rear on the same side to see if one looks low) is worth doing every time you approach the car. However, with modern low-profile tires, differences aren’t as noticeable as they used to be.
Aside from the aforementioned pressure changes due to temperature swings, tires often lose a little bit of air just from normal leakage. Worse is that they can also lose some due to a slow leak from a nail or screw in the tire, or if they suffer a harsh road impact such as from hitting a pothole. Thus, even if you live in a very temperate climate, the pressure can drop over time, which may well be less than a month.
As for an actual tire pressure check, a minimum would be a warm day in late spring and a cold day in late fall due to the change in temperature, keeping in mind what temperatures lie ahead. For instance, if it’s a 32-degree day in late fall and you know zero-degree temperatures are likely coming, you may want to add a few extra psi to compensate. It’s not really a problem if your pressures are a little higher than recommended, as you’ll mostly just have a slightly stiffer ride.