The formula
corrected p = (p + 1.013) × (273.15 + T now) ÷ (273.15 + T reference) − 1.013
Where it comes from
There is air inside a tyre, and air is a gas: at constant volume, pressure rises with temperature. The gas law applies to absolute pressure, not to what the gauge reads, so you add atmospheric pressure before correcting and take it off afterwards. In practice it works out at about one per cent per ten degrees: a tyre set to 2.3 bar in August reads almost 0.2 bar lower on a January morning, and that is where the dashboard warning comes on with nothing punctured at all.
How to work it out by hand
- Add atmospheric pressure, 1.013 bar, to the placard figure
- Multiply by today's temperature in kelvin and divide by the reference one
- Take atmospheric pressure off again
- Add the load increase the placard gives, in proportion to how loaded you are
What is worth knowing
Pressure is always measured cold, meaning the car has stood for at least two hours or has driven less than two kilometres slowly. Rolling warms the tyre and raises the pressure by 0.2 to 0.3 bar: if you inflate hot up to the placard figure, you end up below it once it cools. A tyre twenty per cent below its proper pressure uses more fuel, runs hotter and wears out considerably sooner, with the wear concentrated on the shoulders instead of spread across the tread.