A car carrying a heavy load, or towing, needs more air in its tyres than the same car running light. The recommended pressure rises with weight, which is why nearly every pressure placard gives a second, higher set of figures for a full load.
The laden figure
Alongside the normal pressure, the placard lists a laden figure, the pressure to use when the car is heavily loaded with passengers and luggage, or towing. The extra weight has to be carried by the air in the tyres, and without the extra pressure the tyres flex too much, run hot and wear at the edges, exactly as if they were under-inflated.
The raise usually applies to the rear tyres, where most of the extra load and the towball weight sit, though the placard is the guide to which axle and by how much. The pressures should be set back to normal once the load is gone.
A caravan or trailer is separate
The towing car's pressures are only half the job. A caravan or trailer has its own tyres with their own correct pressures, and these are not related to the car's figures. They are worked out from the maximum load and pressure marked on the trailer's tyres and the actual weight being carried, and they often run close to the tyre's maximum.
These tyres are easy to neglect because a caravan stands unused for long spells, but their pressure matters just as much. It is also worth knowing that when towing on public roads, the trailer's tyres must be legal too, defects on them are an offence in the same way as on the car.
Getting it right
Before a loaded trip or a tow, the routine is: set the car to its laden pressures, check the caravan or trailer tyres against their own figures, and include the spare on both. When buying tyres for a vehicle used to tow or carry loads, getting the load rating right matters as much as the pressure, since the pressure only works if the tyre is built for the weight; the load rating sits with each tyre at online sellers such as Tyres.co.uk, so a set rated for the job is easy to pick.
From the workshop: caravan tyres are the ones that catch people out. Plenty of tread, looks fine, but it has sat at the wrong pressure for three winters and the rubber has had it. Set them to their own figure and check the date code before a big trip.
Sources and accuracy. The laden-pressure and trailer guidance here reflects manufacturer and towing-body guidance at the time of writing; the car's placard and the trailer maker's figures are definitive. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Should I increase tyre pressure when carrying a heavy load?+
Yes. Most cars have a higher laden pressure on the placard for a full load of passengers, a loaded boot, or towing. The extra weight needs more air to support it, so the tyres should be raised to that laden figure and dropped back afterwards.
Which tyres need more pressure when towing?+
Usually the rear tyres of the towing car, since that is where the extra weight and the towball load sit. The placard's laden figures show which axle to raise and by how much. A caravan or trailer's own tyres are set separately to their own pressures.
Does a caravan or trailer use the same pressure as the car?+
No. A caravan or trailer has its own correct pressure, worked out from the maximum load and pressure marked on its tyres and the actual loaded weight. These often run close to the tyre's maximum, and are quite different from the car's figures.
Do trailer tyres have to be legal too?+
Yes. When towing on public roads, the tyres on a caravan or trailer must meet the same standards as the car's, and defects on them are an offence in the same way. They are easy to overlook, so they deserve the same checks.