Tyre pressure is not fixed, it rises and falls with temperature. Air expands when warm and contracts when cold, so the same tyre reads higher on a hot afternoon than on a frosty morning, with nothing actually wrong.
The rule of thumb
As a guide, every 10°C change in temperature shifts tyre pressure by about 1 to 2 PSI, down as it gets colder, up as it gets warmer. Across the swing from a mild autumn to a hard winter, that can add up to a noticeable drop, enough to affect grip, wear and fuel use.
This is separate from the heat of driving. Rolling along a road warms the tyres and can add 5 to 10 PSI on top, which is exactly why pressures are always set cold. The recommended figure is a cold one, measured at the prevailing outside temperature.
The cold-morning warning light
A common winter sight is the pressure warning light coming on during a cold snap, often first thing in the morning. Much of the time this is simply the temperature at work, not a puncture: the overnight cold has pulled the pressure below the level that trips the warning. It may clear as the day warms and the tyres heat with driving.
That said, the light should never be ignored. The right response is to check the pressures cold and top them back up to the recommended figure, and if one tyre is well down compared with the others, to look for a slow leak rather than blaming the weather.
Setting pressures by season
The practical approach is straightforward:
- Set pressures cold, at the current outside temperature, to the placard figure
- Check more often in winter, as each fall in temperature lowers them further
- Don't reduce a warm reading to the cold target, a tyre that is a few PSI high after driving or in the heat is normal
Cold-weather tyres do not escape this, by the way: a winter or all-season tyre loses pressure to the cold just as a summer one does. Fitting winter tyres, a set the online tyre shops let you find by size, Tyres.co.uk among them, improves grip, but the pressure still needs checking through the cold months.
From the workshop: every autumn the warning lights start, and most are just the cold. We tell people to top them up to the door figure on a cold morning and check again in a fortnight, rather than panicking about a puncture that usually is not there.
Sources and accuracy. The temperature rule and guidance here reflect manufacturer guidance at the time of writing and are approximate; the car's recommended cold pressures are the figure to work to. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
Why does tyre pressure drop in cold weather?+
Because air contracts as it cools. As a rule of thumb, every 10°C fall in temperature drops tyre pressure by about 1 to 2 PSI. A cold snap can take pressures noticeably below the recommended figure, even with no leak at all.
Why does my tyre pressure light come on in the morning?+
Cold overnight temperatures lower the pressure, and the warning light reacts to that drop. It often clears as the day warms and the tyres heat with driving, but it is still a prompt to check and top up to the recommended cold figure.
Should I add air in winter?+
Often, yes. As temperatures fall, pressures drop below the placard figure, so topping back up to the recommended cold pressure keeps grip, wear and fuel use right. Check more often during cold spells, as each drop in temperature lowers them further.
Is higher pressure in summer a problem?+
Warm weather and driving both raise pressure, but the recommended figure is a cold one that already allows for this. A warm tyre reading a few PSI high is normal and should not be let down to the cold target. Only set pressures when the tyres are cold.