Cold tyre pressure

Cold pressure is the number the placard is written for. It assumes the air inside is close to ambient. After twenty minutes on a motorway, flex and brake heat raise that air. The gauge climbs 0.2–0.4 bar on a passenger tyre, more on a loaded van in July.

That rise is not a bonus you should dump. If you bleed a hot tyre down to 2.4 bar after a trip, you may wake up at 2.1. Under-inflation then makes even more heat the next day.

A short trip to the shops does not fully heat a tyre. A long descent with heavy braking does. If you only have a hot reading, fill only if you are still below placard while hot — that tyre was dangerously low when cold.

Track-day crews set a cold start pressure so that hot pressure lands in a window. Street placards are cold-only. Do not borrow the race method for a school run.

Dash readouts on newer cars are often warm and rounded. Use them as a leak alarm, then confirm cold on the driveway.