Bit of a numpty question this.

Spend most of my time riding bikes, on which the tyre pressure tends to be determined by the tyre manufacturer. So on the car there is a stcker that says what the pressures should be?

Then i hear that pressure should be raised for high speed runs or for loads? That under inflated tyres are bad.

So what decides the correct tyre pressure

Car Bibles : The Wheel and Tyre Bible Page 1 of 2 Try this site it explains most things to do with tyres, Or may help to answer your question.

many things really, things like load, if your carrying alot of weight, eg passengers or bagage, then you want slightly higher pressures, also weather, if its icy then they reccoment running your tyres slightly lower pressure to help increase tyre contact with the road.

On my octavia, there is a sticker on the inside of the fuel cap gibing pressures for the number of passengers your carrying.

Not much of an explanation, but I stick with the recommended pressures most of the time. I raised the rear's about 1-1.5psi when I went on holiday last year and didn't notice any abnormal wear so I guess it was about right.