Tire Pressure Explained: PSI, Load and Tire Size Changes

Tire pressure is the air pressure inside the tire, almost always measured in PSI (pounds per square inch) or bar. The figure stamped on your sidewall is a maximum — it is not a recommendation. The pressure to actually use is printed on the vehicle's tire and loading placard, normally inside the driver's door jamb.

Pressure matters because it directly controls load capacity, contact patch shape, rolling resistance and heat build-up. Too low and the sidewall flexes excessively, the shoulders wear out, fuel economy drops and the tire can overheat at speed. Too high and the centre of the tread wears prematurely, ride quality suffers and grip in the wet decreases.

Tire pressure recommendations are always cold — measured before driving or several hours after the car has stopped. Pressure rises 1–2 PSI for every 5 °C of heat build-up, which is normal. Never bleed off hot pressure: you will end up under-inflated when the tires cool again.

Changing tire size can change the right pressure because load capacity scales with the tire's air volume and contact patch. A larger tire of the same load index can usually carry the placard load at slightly lower pressure; a smaller tire generally needs more pressure to match the original capacity. Online calculators give an estimate at best.

The authoritative source is always the combination of your vehicle's placard load and the specific tire manufacturer's load/inflation table for the new size and load index. These tables account for sidewall stiffness, ply rating and bead profile that no proxy formula can capture.

Underinflation is the most common cause of catastrophic tire failure on the road. Overinflation is less dangerous but still degrades grip, comfort and even tread life. Check pressures at least monthly, and always before a long trip or a heavy load.