Check your maintenance handbook or on the edge of your driver-side door for the pressures recommended by your car's manufacturer. Depending on the weight distribution of your vehicle, the pressure may be different at the front and rear. The load also influences the recommended pressure: the fuller your vehicle is, the more you'll have to inflate the tyres.
Cold pressure:
Ideally, you should check and adjust the cold pressure, as driving causes the tyre to heat up and increases the pressure. If you do a cold test, inflate to the pressure indicated by your car's manufacturer.
'Hot' pressure:
The pressure indicated by the manufacturer is for a cold tyre, i.e. a tyre that hasn't just been driven on. When your tyres cool down after use, they will lose about 0.3 bars of pressure. Therefore, you should over-inflate your tyres by 0.3 bars when they're hot in order to get the correct pressure.
In winter:
You can over-inflate by 0.2 bars in winter weather conditions, because the cold reduces the pressure of your tyres.
At altitude:
No modification is required, the cold that decreases the pressure is made up for by the altitude that increases the pressure. These two effects cancel each other out.
On the motorway:
You can increase your pressure by 0.3 bars if you often do long distances on fast roads. This will help the tyre to better resist the crushing effect on the tread due to the high number of revolutions per minute at high speeds. This recommendation is particularly valid for lovers of high-speed driving.
