And you just hit on one issue with using nitrogen to fill your tires, Busa! Unless the tire is mounted in a nitrogen environment, or drained and refilled several times over, there'll always be a substantial amount of atmospheric air left in it. And getting a tire properly filled with nitrogen isn't cheap, initally... Just to clarify some misconceptions: Nitrogen fill is used exclusively in aircraft and racing because it is, as TonyMac said, inert. But this isn't due to flammability; it's because nitrogen will not expand or condense any appreciable amount in those applications, holding the same pressure regardless of temperature change. Also, because it's inert, it won't react in any way with the rubber compounds or metals: No degradation or oxidation. Nitrogen itself is fully inert and will not react with other substances. The gas molecules themselves are much bigger than, say, oxygen or carbon dioxide (the two other most prevalent gases found in the atmosphere), heavier as well, so the tire will "bleed" pressure at a much slower rate over time, usually well beyond the life of the tire. For this same reason, it's used by several manufacturers of earthmoving equipment. Barring a puncture or other leak, you'll never have to reset the tire pressure! There really are some great advantages to using nitrogen to fill your tires! But as I mentioned before, the initial cost of properly purging and inflating may be... daunting.
