Although it’s often overlooked, checking and adjusting your car’s tire pressure is an important part of safe driving. Particularly if underinflated, improper tire pressure can adversely affect handling and stopping distance, make the tires more prone to blowouts, affect ride quality and result in uneven tread wear.
Related: What Should My Tire Pressure Be?
The manufacturer’s recommended tire pressures for your car are listed on a sticker affixed to the driver-side front door jamb. Proper tire pressures are also listed in the owner’s manual, though reading the door-jamb sticker is probably handier. A common range for passenger cars is 30-35 pounds per square inch, though the tire pressure range for SUVs and trucks may be higher. Note that some cars have different recommended tire pressures for the front and rear tires, and some may have higher pressures listed for higher driving speeds or when carrying a heavy load.
It’s also important to not inflate the tires to the pressure listed on the sidewall of the tire itself, as that figure indicates the tire’s maximum safe pressure, which is usually much higher than the manufacturer’s recommended pressure.
Why Tire Pressure Can Get Low Without Having a Flat
One important thing to keep in mind is that tire pressure varies with temperature. That’s because the air that fills the tire expands as it gets hotter (thereby raising the pressure) and contracts when it gets colder (lowering the pressure). That variation in temperature can come from a change in ambient air temperature, having the tire sit in the sun or driving, which heats up the tire and, thus, the air inside. Therefore, the ideal time to check tire pressure is when the tire hasn’t been driven on in a while and has been sitting outside in the shade (even being inside a garage can make it hotter).
In terms of air temperature, the general rule is that tire pressure changes by 1 psi for each 10-degree change in air temperature. That means that if you filled your tires to the proper pressure on a 90-degree summer day, they’d be about 9 psi lower on a zero-degree winter day. Since a vehicle’s proper tire pressure is the same in all seasons, that means a tire can become dangerously underinflated in cold weather. (It also means it can become overinflated in hot weather, but that’s not usually as bad.) Furthermore, even good tires can leak out a little air over time.
The opposite can also be true: If you inflate your tires when it’s cold out, they could end up being overinflated when it gets hot again. You can let out air by pressing on the tiny pin in the center of the tire’s valve stem. Often, your fingernail will work for this, and some tire pressure gauges have a built-in means to do so.
Don’t Forget the Spare
You should also check your vehicle’s spare tire periodically as air can leak out over time — that is, if your car has one (some just include an inflator kit). Note that some of the small, skinny, “doughnut” spare tires typically require much higher air pressure than the car’s regular tires, often around 60 psi.
Also note that many full-size spare tires are hidden beneath the rear of the vehicle where they’re hard to get to, and checking the pressure may be very difficult. If that’s the case, at least try to press hard on the tire’s sidewall to see if it’s firm; you can compare the feeling to that of doing the same with one of your regular tires.
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Why Is My Tire Pressure Warning Light On?
Both a temperature change and a slow leak can lower tire pressures enough to trigger a warning via the car’s tire pressure monitoring system, which displays via a warning light on the instrument panel. (Tire pressure monitoring systems have been mandated on new cars since the 2008 model year, with many vehicles getting them well before that.) The light is programmed to go on when tire pressure is about 25% below the recommended pressure. People often see the light come on during the first really cold day in the fall, inciting just a bit of panic, then notice it goes out after driving awhile; that’s the aforementioned temperature variations at work.
Checking Tire Pressure
There are various types of gauges you can buy to check tire pressure.
Perhaps most common is the stick type, which looks like a pen. When you check the pressure, the “stick” shoots out of the bottom of the pen. These are usually quite inexpensive, but their angled tip that goes over the tire’s valve stem and the small markings on the stick can make them tough to get a leak-free connection and see the resulting reading. If you hear a hiss when you press the gauge’s tip onto the valve stem or see a very low reading on the stick, you probably aren’t getting a good seal.
Somewhat easier to use is a gauge with a tip that presses straight onto the stem and gives its reading on a round dial. These gauges sometimes have flexible hoses between the tip and the dial that make them more adaptable to different applications, but they may require two hands to use. A variation on this design uses a screw-on tip rather than a press-on tip. That ideally makes for a more secure connection and a more accurate reading, though sometimes tightening the screw-on tip can be tricky, allowing some air to leak out. Another variation uses a built-in clamp on the tip. You press down on a little lever with your thumb, press the tip onto the stem and release the lever to clamp the tip onto the stem.
There are also digital gauges that can be very accurate and easy to use. However, these run on small batteries that can go dead, rendering them useless until you replace the batteries, which can be a hassle.
Gas stations used to routinely have air pumps, but many don’t anymore. That means you may have to stop at a tire store or use your own.
Personal pumps can vary from manual hand-operated ones (like the tall, skinny kind or foot-operated versions) to electric ones you plug into a wall outlet, your car’s 12-volt power outlet or that run off their own battery. Those that run off electricity are often referred to as tire inflators. (Larger ones that plug in and have a tank attached are known as air compressors, which are not what we’re talking about here.)
Whichever kind you buy, it’s recommended that you pay a bit more to get one with a built-in gauge; you’ll thank yourself later. Ditto if you can find any variety of an electric one that has an automatic shut-off when it reaches a dialed-in pressure; this way, you can connect the hose, dial in the proper pressure, turn on the pump and walk away; the unit will automatically shut off when the proper pressure is reached, which is particularly handy if you have to pump in a lot of air and it’s cold outside.
Electric tire inflators that plug into a wall outlet are usually fine for your garage, but they don’t do you much good if you don’t have a wall outlet to plug them into. Those that plug into your car’s power outlet can be used anywhere the cord will reach, but the cord can be a hassle. Battery-operated ones are the handiest, but they won’t work if you fail to keep the battery charged. (Most can be charged through a wall outlet or the car’s power outlet, but they sometimes require a charged battery and can’t be run while they’re plugged in.)
While checking and adjusting tire pressures can be a pain, it’s important to keep your tires at the proper pressure — not only for safety, but also for ride comfort and longer wear. It may be worth thinking things through and spending a little more to make the procedure as easy and convenient as possible, particularly if you have to do this all yourself rather than rely on a gas station.