Filling automobile tires with nitrogen is considered safe. Nitrogen has been used to fill airplane and racing car tires for many years. Supporters of nitrogen use claim that it prevents corrosion and holds pressure longer than compressed air.
Nitrogen is used in airplane and racing car tires to help eliminate moisture and control temperatures. The theory of using nitrogen in normal cars is that the cooler temperatures eliminate pressure fluctuations. This results in improved fuel economy and prevents wheel rot. The use of nitrogen has been shown to slightly improve temperatures, but it is highly debatable between experts if the advantages of nitrogen outweigh the increased price and inconvenience.