What tyre pressure actually is

Tyre pressure is the force of air (or nitrogen) inside a closed carcass, measured in bar, pounds per square inch (PSI), or kilopascals (kPa). Most of the vehicle’s weight rides on that compressed gas, not on the rubber. If the number is wrong, the contact patch changes shape, heat builds, and the tread wears in a pattern you cannot rotate away.

A British family hatch typically lives between about 2.2 and 2.5 bar (32–36 psi) when cold. Vans and C-type tyres sit higher. Space-savers are a different class — often 4.1 bar (60 psi). None of those figures come from guessing. They come from the vehicle maker’s load work, printed on the tyre-information placard.

Pressure is not a comfort dial. Dropping 0.3 bar to “soften” a 19-inch plus-size wheel unloads the centre of the tread, loads the shoulders, and raises running temperature. Adding 0.4 bar because the sidewall says 3.5 bar does the opposite.

The useful measurement is cold: the car has stood at least three hours, or covered less than about a mile. Heat from the M1 will add several psi. That rise is expected. Bleeding a hot tyre down to the placard leaves it low on a frosty morning.

This gazette stores a catalogue band in PSI and bar for each model, then estimates nearby sizes. Those figures are a map. The legal setpoint for your VIN is still the sticker on the driver’s door pillar.