Originally Posted by spearfish25

There are certainly a lot of completely wrong answers in this thread thus far.

Nitrogen is an inert gas and as others have said, 'air' is 78% nitrogen. Removing the oxygen and insignificant other gases from the tire inflation composition won't do much other than minimize oxidation to the surfaces within the tire. As Chris has noted, the weight difference between air and nitrogen alone will be negligible in our tires. Molecular size is also not a factor for tire inflation...tires are not selectively permeable membranes that would leave you with pure nitrogen as the oxygen diffuses out more quickly (very funny idea though). Nitrogen's molecular size is nearly identical to that of oxygen anyway.

The ironic part of all this is that people actually pay more money (or any money, if you will) for a nitrogen fill. In SCUBA diving, we pay a small amount more to minimize the composition's % of nitrogen by adding more oxygen to the fill. This blend is called Nitrox and usually it's only a couple dollars more expensive...and this is a filtered, clean blend that we are actually breathing. Paying more for an abundant gas that gives no benefit and doesn't need to meet any cleanliness guidelines is really not necessary.