<P> A tire - pressure gauge, or tyre - pressure gauge, is a pressure gauge used to measure the pressure of tires on a vehicle . </P> <P> Since tires are rated for specific loads at certain pressure, it is important to keep the pressure of the tire at the optimal amount . Tires are rated for their optimal pressure when cold, meaning before the tire has been driven on for the day and allowed to heat up, which ultimately changes the internal pressure of the tire due to the expansion of gases . The precision of a typical mechanical gauge as shown is ± 3 psi (21 kPa). Higher precision gauges with ± 1 psi (6.9 kPa) uncertainty can also be obtained . </P>

Instrument used to measure air pressure in tyres