PSI (pounds per square inch)

PSI stands for pounds per square inch, an imperial unit of pressure. Tyre pressures in owner's manuals, on tyre sidewalls and on many gauges are often shown in both psi and bar (or kPa), the metric units used in Europe.

Useful conversions: 1 bar ≈ 14.5 psi, so 2.5 bar ≈ 36 psi and 2.9 bar ≈ 42 psi. 100 kPa = 1 bar. The pressure printed on the tyre sidewall is the maximum the tyre can hold, not the recommended riding pressure; use the bike maker's figures.

Check pressures cold, before riding, at least every couple of weeks. Increase them for a passenger or luggage as the manual says. Off-road riders lower pressures for grip, often to around 0.8–1.0 bar, but should then use heavy-duty tubes or mousses to avoid pinch flats.