How It Works: Checking tire pressure

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Everyone knows it’s your tires touching the road, but what you’re really riding on is air. Not only is air pressure the only thing keeping your tire on the rim, but having too much or too little can affect tire wear, handling and even your fuel economy.

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It’s easy to check tire pressure, and the gauges aren’t necessarily all that expensive. It’s recommended to check once a month – including your spare, since it won’t do you much good if it’s flat – because even if they don’t have obvious leaks, tires are permeable and slowly lose a bit of air over time. And unless it’s really down on air, you can’t tell if a tire is low just by looking at it.

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How It Works: Checking tire pressure Back to video

Tire pressure monitoring systems (TPMS) are not a substitute for checking tires regularly. They’re mandated on all new vehicles sold in the United States but are not required in Canada, although many new cars here come with them anyway — and they’re on all vehicles equipped with run-flat tires, which generally give little indication otherwise if they lose all their air.

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Depending on your system, you may get a warning that a tire is low, without specifying which one it is, while others give you a pressure readout for each. But the warning only triggers when a tire is at least 25 per cent lower than its recommended pressure, and it could be wasting fuel well before that.

An underinflated tire creates more rolling resistance, and the engine has to work harder to overcome it. It’s estimated that some 70 per cent of vehicles in Canada have at least one tire that’s below its recommended pressure.

Some TPMS use pressure sensors inside the tire, either strapped to the rim or as part of the tire valve, while others use wheel speed sensors – part of the anti-lock brake and stability control systems – to determine if one wheel is rolling at a different speed than the others, which could indicate a low tire. Some auto manufacturers also use the in-tire sensors as part of an air filling alert system, such as Nissan’s Easy-Fill, which chirps the horn when you’ve added the right amount of air.

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How to check your tire pressure

There are three types of gauges for checking pressure: stick, dial and digital. Stick gauges are the least expensive, but they and dial gauges can sometimes be inaccurate, so beware the rock-bottom cheapest in both. Digital gauges are usually accurate, but they cost more and require a battery.

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To use a gauge, remove the cap on the tire valve. (The caps don’t hold in the air; their job is to keep out dirt and moisture.) Place the end of the gauge over the valve and then firmly press it on. Do this quickly and evenly, to avoid losing air. There’s a small stem inside the valve and the gauge pushes on this, opening the valve. Air exits the valve but only gets as far as the gauge, which measures the pressure. When you remove the gauge, the stem rises and the tire valve seals shut.

On a stick gauge, the pressure pushes out a calibrated stick, and the highest number visible on it is the tire’s pressure. Dial gauges spin a needle to show the pressure number, while digital gauges have a readout. Pressure is measured in kilopascals (kPa) in metric, but many Canadians still use the Imperial measure of pounds per square inch (psi).

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Tires should be checked when they’re at ambient temperature, and you should wait until the vehicle has been sitting for at least two or three hours, or even overnight. The pressure rises when a tire gets hot from driving, and you can get a false reading.

How to fill your tires properly

It’s important to inflate to the recommended pressure, which can vary depending on the type of vehicle and the tire. This number is on a placard that’s usually found in the driver’s door jamb, or sometimes inside the glove box or the fuel door. Don’t use the number that’s cast into the tire’s sidewall: that’s the maximum it can hold. If you replace your tires with a different size, ask the dealer what pressure they should use.

When filling your tires, it’s best not to trust the reading on the air pump. Instead, inflate the tire and then check it with a gauge. If you’ve put in too much, use the end of the gauge, or a tool such as a screwdriver, to press on the stem inside the valve to let some out.

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You might have heard about filling tires with nitrogen. If a tire contains it – sometimes from the factory, or if a shop has used it when changing tires – it will often have a green valve cap. Proponents say that nitrogen-filled tires aren’t as prone to gradual pressure loss, but it’s negligible: in a year-long test, Consumer Reports found only a 1.3-psi loss difference between air and nitrogen.

And while nitrogen is not as easily affected by temperature changes, the benefits are most noticeable in racing and airplane tires, not everyday vehicles. Fill your tires with air, check them regularly, and save yourself the cost of nitrogen.

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