Hazards Driving with Under-Inflated Tyres.

Why tyres lose pressure

Some pressure loss is normal. Air slowly permeates the rubber, the valve and the seal between the tyre and the rim, and a drop in air temperature lowers the reading without any air actually escaping. A tyre can also pick up a slow puncture from a nail or screw that seals around itself and leaks over days rather than minutes.

This is why a monthly check matters even on a car that seems fine. A tyre that has quietly lost a third of its pressure still looks close to normal, but it behaves very differently on the road.

What happens when a tyre runs low

An under-inflated tyre flattens more than it should where it meets the road. The sidewall flexes further with every rotation, which builds heat inside the tyre — and heat is what damages the casing and, in the worst case, leads to a sudden failure at speed.

• Longer stopping distances and vaguer steering, especially in the wet
• Uneven wear — the tread shoulders wear faster than the centre
• Higher fuel use, because the engine works harder against the extra rolling resistance
• More heat, raising the risk of a blowout on a long, hot or heavily loaded run
• A loaded vehicle or trailer makes all of the above worse

How to check pressure properly

A pressure reading is only meaningful when the tyre is cold — before driving, or after the vehicle has been parked for a few hours. Driving warms the air inside and raises the reading, so checking straight after a trip gives a false high number.

• Use the pressures on the vehicle's tyre placard, not the number on the tyre sidewall (that is a maximum, not a recommendation)
• Use a gauge you trust; many service-station gauges are inaccurate
• Check all four tyres and the spare
• Use the higher laden pressure when carrying a load or towing
• Re-check about a week after adjusting, in case a valve or tyre is leaking

Signs to act on

Some of these you can see in the driveway; others you only notice through the steering wheel or in how the car sounds as you drive.

• One tyre that always needs more air than the others — have it checked for a puncture or valve leak
• A tyre that looks visibly flatter than its pair
• Vibration, pulling to one side, or a change in road noise
• Tread worn more on both edges than in the middle