Warning lights for tyre pressure appear in clusters during the first cold weeks of autumn, on cars with no puncture and no leak. The cause is the behaviour of gas in a fixed volume.
Pressure and temperature move together
The air in a tyre occupies an essentially fixed volume. When it cools, the molecules move more slowly and strike the inner surface with less force, so the pressure falls.
The relationship is close to linear across ordinary conditions, and a drop of ten degrees produces a fall of a few per cent in gauge pressure.
A seasonal shift from a warm summer evening to a cold autumn morning can therefore take a correctly inflated tyre below its warning threshold without any air escaping.
Warning systems detect the change in different ways
Direct systems use a sensor inside each wheel that measures pressure and transmits it, giving a genuine reading that responds to temperature immediately.
Indirect systems infer pressure from wheel speed data, since an underinflated tyre has a slightly smaller rolling radius and turns marginally faster.
Indirect systems compare wheels against each other, so a uniform drop across all four caused by cold weather may be detected late or not at all.
Driving warms the tyre and masks the loss
Flexing of the sidewall generates heat, and a tyre warms substantially in the first several miles of a journey, raising its pressure as it does.
A warning that appears at the start of a cold morning may therefore clear itself once the car has been running, which convinces some drivers that nothing is wrong.
The underlying pressure is still low when cold, and the tyre is running underinflated for the portion of every journey before it warms up.
Underinflation causes more than fuel consumption
A soft tyre flexes more at the shoulders, generating heat that concentrates there, and sustained running in that condition damages the structure of the tyre.
The contact patch changes shape, loading the outer edges and unloading the centre, which alters braking distances and how the car responds to steering inputs.
Wear follows the same pattern, so the tyre records the problem visibly at the shoulders long before the driver notices any change in behaviour.
Checking cold is the only meaningful measurement
Manufacturer figures are specified for a cold tyre, meaning one that has not been driven for several hours or has covered only a very short distance.
Checking after a motorway run gives a reading several units higher, and setting to the placard figure at that point leaves the tyre underinflated once it cools.
Loading changes the requirement as well, since most placards list a higher figure for a full load, and that figure is frequently overlooked before a long journey.