Recently, I went tire shopping for my Mazda 626 and came across the idea of nitrogen-filled tires. A brochure (from a company that shall remain unnamed) I picked up at the tire shop claimed that:
2. The moisture in "wet oxygen" deteriorates rubber, which dry N2 does not.
3. N2 is a bigger molecule than O2 and therefore N2 leaks 3-4 times slower.
4. This loss of tire pressure leads to reduced fuel economy, reduced tire life and increased tire maintenance.
Out of these claims, #3 is often misunderstood. Oxygen molecule (O2) has a molecular weight of 32 while nitrogen molecule (N2) has a molecular weight of 28 (amu). It is clear that O2 is a bigger molecule compared to N2. However, the effective size of the oxygen molecule (called the kinetic diameter) during diffusion through the rubber tire to the surrounding air is smaller than N2. Some of the concepts involved are explained here.
It might be true that oxygen leaks out 3-4 times faster than pure N2, but the air we breathe is mostly ~79% N2 and 21%O2. Therefore, the actual beneficial effects of filling nitrogen in the tire are actually lesser (since oxygen accounts for only 21% of the tires volume, the rest being N2). If we take these two values and assume that O2 leaks 4 times faster than N2 out of a tire, then the tire filled with N2 will hold its pressure for ~40% longer time, compared to the air-filled tire. Ultimately, the choice of filling N2 in tires will come down to personal preference. If you are a normal car user, check tire pressures regularly and ensure that the tires remain properly inflated, then N2 filling may not be the best choice (considering that one has to pay for this). However, if you want the comfort of knowing that your tires will remain inflated for longer time periods, N2 filling may be an option.
Some more links related to the effects of nitrogen filled tires: