Check the spare, and fill to the door sticker

If you have ever stood at an air pump and wondered which number to trust, there are two on offer, and they are not the same. One is moulded into the side of the tire in large letters. The other is on a sticker you probably have not read. And somewhere in your car there may be a spare you have not looked at since you bought it.

If your car came with a sealant kit and no spare, only the first half of this page is for you. The last section says what to look for.

Fill your tires to the number on the sticker in the driver’s door jamb or in the owner’s manual, and check the spare’s air at the same time.

Which number is the right one

The US Department of Energy’s fuel economy site says the proper pressure for your vehicle is usually on a sticker in the driver’s door jamb or the glove box, and in the owner’s manual, and it says not to use the maximum pressure printed on the tire’s sidewall. A vehicle maker’s owner’s manual says the specifications are on a label on the driver’s side center pillar. Many people say the same thing: the sidewall number is the most that tire can hold. One person points out that the tire maker does not know how heavy a given car is, so the car maker sets the number to use. Some people point out that the gap is usually large, with the car maker’s figure in the 30s (psi) and the tire’s own maximum around 50.

Some people clarify that filling to the sidewall number is a limit, not a trigger for an instant burst. That is no reason to use it. One person says the cost is a harsh ride and less grip, and another warns that it wears the middle of the tread faster.

The number on the sticker is for the tires the car came with. Some people warn that if you changed the tire size or the load rating, the sticker may no longer fit, and that there is a chart for working out the new figure from the original load capacity. One person points out that unless the change is drastic, the sticker number is still the one to follow.

Check them cold

A vehicle maker’s owner’s manual says to check when the tires are cold: the car has not been driven for at least three hours, or has been driven less than a mile. It also says warm tires normally read 4 to 6 psi higher than the cold figure, and not to let air out of warm tires to bring the reading down, or they will end up under-filled. Some people say pressure also falls as the weather gets colder, so check more often when the temperature drops.

The same manual says severe under-inflation, 10 psi or more, risks heat build-up, a blowout and the tread coming apart. It says under-inflation also causes excessive wear, poor handling and worse fuel economy. Some people answer a suggestion to let air out of road tires for snow by saying it does not help on standard tires, and the manual’s warning above applies: stay at the sticker number. Some people add that the warning light on the dashboard may only come on when a tire is far too low, so a gauge is still needed. Some people say the gauges at gas stations are often broken or missing, and that built-in ones can read inaccurately. One person suggests owning a pump and a digital gauge so you can check for yourself, and another says a stick gauge can lose its accuracy if it has been knocked about.

The spare: check it before you need it

Many people say a spare slowly loses air and can be flat by the time you need it. A vehicle maker’s owner’s manual says to check the pressure of your spare each time you check the others. One person says doing it monthly is an easy habit, and that it can also show up a slow leak elsewhere.

The spare may not use the same number. One person says to put some air in and leave it at that. Others say you should check the spare’s own pressure first, because it may differ from the road tires, and not just add air. A vehicle maker’s owner’s manual says its compact spare should be filled to 60 psi, and that it must not be driven over 50 mph (80 km/h). Your own manual, or the label on the spare, has the number for yours.

One person says a spare can age out even when it holds air, and that if the car is over ten years old, the spare’s rubber may have degraded. Another says that when buying a used car, check the spare holds air and the jack and wheel wrench are present and working.

There is a page here on changing the tire itself: loosen the lug nuts before you lift the car.

What to fill it with

Many people recommend carrying a small air compressor in the car, the kind that plugs into the lighter socket, so you can fill a tire or the spare without depending on a stranger’s pump. Some people add that a digital inflator gives a precise reading. People disagree about which is better: some prefer electric ones for the speed, and others prefer a manual pump because it costs less and is less likely to fail. Some people say to test the compressor before an emergency, because cheap ones can fail from overheating or a defect. Some people say portable compressors can break when they are needed most, and one person says a small one may not fill a completely flat tire in one run because of the heat. One person says a hand pump takes about fifteen strokes per psi, so it is a backup for a short distance and not a plan.

Some people say that at some stations the air is free if you buy fuel, or the attendant will turn the pump on if you ask. One person says you let air out by pressing the pin in the middle of the valve. One person says to look for a slow leak by brushing soapy water on the valve and watching for bubbles.

If you hear a sound like a zipper while you fill a tire, some people say to stop at once and move away. One person warns that a tire driven flat for long enough to damage its inside needs a safety cage to be filled.

If your car has no spare

Not every car has one. Some people say some come with sealant and a pump rather than a wheel, and some with run-flat tires. Some people say there is room for a full-size spare on some cars and not on others. One person warns that sealant can ruin the pressure sensor in the valve. Look now, in the trunk or under the floor, not at the roadside. This page does not cover what to do with a flat when there is no spare.

Who this page is not for

If you are at the roadside with a flat right now, the page on changing the tire comes first, and do not stay in or near a lane of traffic. If your car has no spare, the spare half does not apply to you. If a warning light is on, this page does not say what it means; your owner’s manual does. If you tow, carry heavy loads, drive heavy trucks or have fitted wheels and tires that are not the size the car came with, the sticker may not be the whole answer, and this page does not cover it.