Nitrogen, the main constituent of air, has the same basic physical properties as air and responds in exactly the same way as air when the temperature and pressure vary (over the ranges that would be found within a car's tyres). It would be used in tyres because it contains no oxygen, carbon dioxide or water vapour - this means that steel rims and valve parts (if any) do not rust on the inside and that the internal surfaces of the rubber tyre are protected from the ageing effect of those gases, too.