ScottG wrote:
Nitrogen will not make any difference because most of the loss is where the tire seals to the rim and that space is relatively large.
Besides this, normal plain air is mostly nitrogen anyhow—roughly 80% by volume. The rest is mostly oxygen, with only a very small portion of carbon dioxide and other gasses.
Oxygen molecules are very slightly smaller than nitrogen molecules, and so do permeate through rubber slightly more quickly, but diffusion of the gasses though the rubber of the tire is slow enough that it's senseless to concern oneself with the difference. It's still necessary to check tire pressure and adjust it as needed on a regular basis regardless.
I was going to suggest that carbon dioxide should be even better than nitrogen, since it has larger molecules, but it turns out that (while the molecules are larger) the chemistry of gasses permeating solids gets rather more complicated and for various reasons carbon dioxide diffuses through rubber fairly rapidly—much more quickly than either nitrogen or oxygen.
