Running car tyres at the correct pressure is important for both safety, economy, and performance. Most modern cars have a tyre pressure plate usually found in the door opening. This plate will indicate the suggested pressures for the different types of tyre that can be fit to the car and the loading. In this example the tyre pressures for the front and back are slightly different depending on whether it has larger or smaller tires fitted and whether the car is fully loaded or just carrying a couple of passengers.
For our car, which has 205/45 R17, with two people on board the front tyre pressures are 2.4 bar or 35 psi. The rear tires would be 2.2 bar or 32 psi. the sizes of the fitted tires can be read from the side of the tire. First two numbers are the width of the tire and an indication of its height, the number after the R is the wheel diameter. Expected pressures shown in red....
Cars that have run flat tires and many modern cars have TPMS (tyre pressure monitoring systems) that can tell when a tire is under pressure. It doesn't record this by looking at the actual pressure but by working out the speed of rotation and the known diameter of the tyre, and the steering angle. Needless to say, the internal tyre pressure warnings should not be ignored. The systems will normally display a service request message and or warning if it detects that the tires are under the normal working pressure.
After tyres are re-pressurised this system is reset and after a short journey new readings will be obtained.
A warning
Like many motorists, I kept a tyre pressure gauge in the glove box to check the garage forecourt pressures as those can often be unreliable. I had a black rubber covered Pumpkin tyre pressure gauge for about 5+ years after which it was contradicting the internal tyre pressure sensors of the car. It had a very easy life living in the glovebox most of the time just deployed every couple of months to check the tyre pressures. It now turns out it may have been providing misleading information for a number of years. By comparison with another pressure gauge it could be seen that the Pumpkin gauge was reading out by about over five psi.
The fault, as can be seen in the second picture, is the pumpkin gauge does not reset to 0 making all readings about 4 or 5 PSI over the actual value.
This should serve as a warning that all tools and instruments that you rely on should be periodically recalibrated against a trusted or another source to ensure accurate operation.