Tyre pressures are quoted for cold tyres and will be specified in your car manual. They may also be shown on a sticker inside the fuel filler cap or driver's door.
Generally two figures will be quoted; one for 'normal' use and a higher figure for full loads. You should make sure to adjust the pressures as required.
If you don't know the correct tyre pressures you can contact the vehicle manufacturer's customer services department.
Correct tyre pressure is important for several reasons:
Tyre life – under or over inflation increases wear
Fuel economy – under inflation increases fuel consumption
Safety – under or over inflation will affect grip and braking performance
Ride – over inflation can result in a harsh, uncomfortable ride
You should check tyre pressures every two weeks when the tyres are cold using a reliable tyre pressure gauge.
At the same time, examine the tyres for any cuts or bulges. If you find any, have them checked out by a professional as soon as possible. A bulge in a tyre indicates internal structural damage so the tyre will need to be replaced.
The tyre must also be replaced if you find cuts deep enough to reveal the internal structure of the tyre.