Why check your tyre pressures?

Tyres lose air even when nothing is wrong with them. A little escapes through the rubber, round the valve and where the tyre meets the wheel: typically around 1 psi a month, and faster when the weather turns cold. You can't see it happening. A tyre that's 8 psi down looks almost exactly like one that's right.

That's why so many cars are running low. When the tyre-safety charity TyreSafe checked, it found 57% of car and van tyres on UK roads were below the recommended pressure, and at least 35% were 8 psi or more under.

1. Grip and braking

Your tyres are the only part of the car touching the road, and each one does it through a patch roughly the size of your hand. The manufacturer's pressure is the one that gives that patch the right shape. Too little air and the tyre squirms and the sidewalls flex: the steering feels vague, the car leans more in corners, and in the wet the tread can't clear water as well, so you stop later.

2. Heat, and the risk of a blowout

A soft tyre flexes more every time it turns, and flexing makes heat. On a hot day, at motorway speed, with the car loaded for a holiday, an under-inflated tyre is working much harder than it was built to. That's how tyres fail at speed.

3. Fuel

A soft tyre takes more effort to roll. The US Department of Energy puts it at about 0.2% worse fuel economy for every 1 psi drop in the average pressure of all four tyres. That sounds small, but tyres 8 psi down cost you around 1.6% on every tank, and TyreSafe has estimated UK drivers waste up to £600 million a year on fuel because of low pressures. For an electric car, it's range you lose instead.

4. Tyre life

Run a tyre soft and it wears on both outside edges; pump it too hard and it wears down the middle. Either way you're buying tyres sooner than you need to. A new pair of tyres costs a lot more than a monthly check.

5. The warning light comes on late

Most cars built since 2014 have a tyre pressure monitoring system (TPMS), and it's tempting to wait for the light. But it's there to warn you of a real loss, usually only once a tyre is well down, not to keep the pressures right. Many systems also only tell you something's wrong, not which tyre. Treat the light as a backstop, not a gauge. (Here's what the light means and how to reset it on your car.)

How often, and how

• Once a month, and before a long trip. Put a reminder in your phone, or let us email you one.
• Check them cold. Before you set off, or after the car has stood for a couple of hours. Driving warms the tyres and pushes the reading up, so a warm check looks better than it is.
• Use the figure for your car, not the tyre. The number moulded on the tyre's sidewall is the most it can take, not what it should run at. Your car's figures are on the label in the driver's door frame or fuel flap, in the handbook, or here by registration.
• Use the fully loaded figures when you're loaded. Full of passengers, a boot packed for a holiday, or towing: most cars have a second, higher set of figures for that.
• Don't forget the spare if you have one. It's no use flat.

While you're down there, look at the tread. The legal minimum in the UK is 1.6mm across the central three-quarters of the tyre, all the way round, and the penalty is up to £2,500 and 3 points per tyre.

Two minutes a month is all it takes. If you're not sure about anything you find, a garage will check and set them for you, often for free.