Increase not only for loading but also for speed. Back in the early 1960s when 'motoring' was still a pleasure, the handbook of my father's Rover 105S instructed - ' for constant speeds over 90mph inflate the tyres by an extra 4 psi.' Forty years later my Focus handbook recommends around an extra 3 psi, depending on tyre, for 'continuous road speed over 100mph'. I wish!

The old MB coupe we have has the tyre pressure chart on the fuel flap as usual, it's opposite to others in that the higher pressure is given as the default figure, unless you read it carefully you wouldn't realise that you are supposed to reduce pressures by 4si below 100mph.