Every autumn, a large number of drivers see a tyre pressure warning for the first time in months. Almost always nothing has gone wrong; the weather changed.

The Physics

Air pressure in a sealed volume falls as temperature falls. The relationship is straightforward and the effect is larger than most people expect.

As a rough guide, tyre pressure changes by around one PSI for every ten degrees Fahrenheit, or roughly five and a half degrees Celsius, of temperature change.

A drop from a mild autumn afternoon to a cold morning is easily enough to take a correctly inflated tyre below the warning threshold.

Set Them at Ambient Temperature

The practical rule. A tyre inflated in a heated garage and then driven into freezing air will lose pressure as it equalises.

Where possible, set pressures with the car at the temperature it will be used in. Where that is not possible, checking again the following morning catches the difference.

A compressor kept in the car rather than in a workshop makes this straightforward, which is a practical argument for a unit that lives in the boot. Ranges covering both cordless and socket-powered units, of the kind at olarhike.net, suit that split: one lives in a cupboard, the other in the car.

What This Means in Practice

• Pressures should be checked more often as the seasons change
• A warning on a cold morning usually means top up, not repair
• Pressures set in summer will read low in winter
• Pressures set in a warm garage read low outside
• A warning that clears as the car warms up is temperature, not a leak

Driving Warms Them Back Up

A tyre gains a few PSI once the car has been driven, which is why a warning light frequently clears itself after ten minutes.

That does not mean the pressure was fine. It means the cold pressure was below the threshold and the warm pressure is above it, and the cold figure is the one that matters.

When It Is Not the Weather

A tyre that needs topping up every couple of weeks has a leak, whatever the season. A slow puncture from a nail is the usual cause and it does not fix itself.

One tyre reading noticeably lower than the others is a leak rather than temperature, since temperature affects all four equally.

Winter Considerations

Cold weather affects more than pressure. Rubber stiffens, grip reduces, and tread depth matters more on wet and icy roads.

A pressure check in autumn is a reasonable moment to check tread depth as well, since the two jobs happen in the same place with the same crouch.

Do Not Over-Compensate

Some drivers deliberately over-inflate in winter to avoid warnings. That reduces the contact patch and grip, in the season where grip matters most.

The correct approach is the placard figure, checked cold, more often.