How to Use This Tool
Enter what the gauge reads now and the temperature it was measured at, then the temperature you care about. The third box tells you what to set today so the tyre is right on a colder or warmer morning.
The law, and the rule of thumb
Pressure and temperature move together, but only in absolute terms. The calculation has to add atmospheric pressure and convert to kelvin first:
P₂ = (P₁ + atmospheric) × (T₂ + 273.15) / (T₁ + 273.15) −
atmospheric
For 32 psi set at 20 °C, that gives 28.81 psi at freezing — a drop of 3.19. The workshop rule of one psi per ten degrees Fahrenheit predicts 3.6 psi for the same change, which is near enough for a forecourt and slightly pessimistic.
Skipping the absolute-pressure step is the common error. Scaling the gauge reading directly would predict a drop of 2.18 psi — about a third too small — because a gauge reads zero when the tyre still contains a full atmosphere of air.
Why the warning light appears in autumn
Direct tyre-pressure monitoring systems typically warn at around 25% below the placard pressure. Set to 32 psi, that threshold is about 24 psi.
A tyre inflated on a warm afternoon and then parked through the first cold night of the season can lose three or four psi to temperature alone. Combine that with the psi or two a healthy tyre loses each month through natural permeation, and the light comes on without anything being wrong.
The correct response is still to check with a gauge, because the light cannot tell you which cause it is. If the pressure comes back to the placard figure after inflating and stays there, it was temperature. If it falls again within days, it is not.
What "cold pressure" on the placard means
It means the tyre is at ambient temperature — parked for at least a few hours, or driven less than a mile or two. It does not mean the weather is cold.
Driving heats a tyre, and a warm tyre can read four or more psi above its cold value. Setting a hot tyre to the placard figure therefore leaves it several psi low once it cools, which is the underinflation the placard exists to prevent. If you cannot check cold, inflate a few psi above the target and re-check in the morning.
Nitrogen does not avoid this
Nitrogen is sold partly on the claim that it is more stable with temperature. It is not: the gas law applies to nitrogen exactly as it does to air, and air is already 78% nitrogen. The measurable difference is that nitrogen is dry and diffuses through rubber slightly more slowly, so pressure falls a little less over months.
The temperature behaviour is identical, and it is not a reason to skip checking pressures.
