Pressure in a car tire

Pressure is proportional to temperature when volume is held constant. An example is the increase in pressure in a car's tires when the car has been driven on the freeway for a while and the tires and air have become hot.

After driving on the freeway for a while, the air in your car's tires heats up from 10 °C to 35 °C. How much does the pressure increase?

The tires may expand a little, but we assume this effect is small, so the volume is nearly constant. From the ideal gas law, the ratio of the pressures is the same as the ratio of the absolute temperatures,

P2/P1 = T2/T1

= (308 K)/(283K)

= 1.09 ,

or a 9% increase.