This popular argument is persuasive: nitrogen filling the tires of your car will minimize air loss, increase fuel economy, decrease rolling resistance, and improve safety. Some automobiles are also sold on valve stems with telltale green caps, showing that the tires have already been loaded with nitrogen. First off, 78 percent nitrogen, 21 percent oxygen and 1 percent other gases are dry air, and some water vapor is almost always present. The nitrogen used to fill tires is a processed gas that is void of all of the normal air impurities, so if you intend to use it, make sure that all the initial air is extracted by the manufacturer to ensure that the tire is truly filled with nitrogen.