The combined gas law applies any time there is a closed system with a fixed amount of gas, including the tires on a car. It expresses the behavior of the gas when temperature, volume or pressure changes, according to About.com.
The combined gas law combines the principles of both Boyle's law, Charles' law and Gay-Lussac's gas law into a single law that states the ratio of the product of pressure and volume to temperature remains fixed for a given amount of a gas.
In the automobile tire, the amount of gas remains constant, yet driving down the road increases the temperature of the tire and the gas inside, thus pressure increases. In addition, the tire has some elasticity, so volume increases slightly. Tire manufacturers take these factors into account as they determine the maximum cold inflation temperature for each tire.
The combined gas law is also important to divers. The container in the case of the diver is the human lung. While under water, the pressure is much greater than in the air. Every foot deeper the diver moves in the water, the greater the pressure becomes. If a diver with full lungs suddenly ascends from the water on a warm day, both temperature and pressure decrease and the volume of the air in the lungs increase rapidly. The diver must exhale the air from the lungs as he ascends to prevent damage due to these changes.