Correct tyre pressure, and how to check it properly

The short answer: your correct pressures are on a plate in the driver's door shut or inside the fuel flap — not on the tyre sidewall. Check them cold, at least monthly, and do not forget the spare.

The number on the tyre is not your pressure

This trips people up constantly. The pressure moulded into the sidewall is the tyre's maximum, not the figure your car needs. Inflating to it will give you a harsh ride, a narrow contact patch and accelerated wear down the centre of the tread.

Most cars list two sets of figures: normal load, and fully laden. If you are running four adults and luggage to the airport, the higher set is the correct one.

Check them cold

Rolling heats the air inside a tyre and raises its pressure, typically by a few PSI. Check after a run and you will read high, let air out, and end up genuinely under-inflated once things cool.

"Cold" means the car has been standing for a couple of hours, or has driven less than a mile or two. If you have to use a forecourt gauge after a longer drive, expect the reading to be a little above the true cold figure.

What under-inflation costs

• Wear on both shoulders. An under-inflated tyre bows in the middle, so the edges carry the load and scrub away.
• Heat. The extra flexing generates heat, and heat is what causes blowouts at speed.
• Fuel. More rolling resistance, more consumption.
• Longer stopping distances, especially in the wet.

Over-inflation is not the safe alternative. It wears the centre strip, reduces the contact patch and makes the car skittish over bumps.

How to actually do it

1. Find the plate in the door shut and note the front and rear figures — they are often different.
2. Unscrew the valve cap and press the gauge on squarely. A hiss means it is not seated.
3. Adjust, then re-check.
4. Replace the valve caps. They keep dirt and water out of the valve.
5. Check the spare while you are there. A flat spare is worse than useless.

A decent pocket gauge costs very little and is more reliable than most forecourt machines, which take a lot of abuse.

If a tyre keeps losing pressure

A tyre that needs topping up every couple of weeks has a problem: a slow puncture, a corroded rim seal, or a perished valve. It is worth finding out which before it becomes a roadside job. See whether a puncture can be repaired, or call 07989 606060.