planet4ever wrote:I had heard that putting in pure nitrogen made the tires lose pressure more slowly, but that must be nonsense, since a molecule of nitrogen has less mass than a molecule of oxygen.
Mass isn't what's important - molecule size is.
Diatomic nitrogen is 3.16 Angstroms long. Diatomic oxygen is 2.96 Angstroms long. The idea is that, since nitrogen gas is slightly larger, it leaks out slower. If this claim were true, we would expect the air in the tire to eventually become almost pure Nitrogen anyway. Repeated refilling with normal compressed air would, over time, result in relatively pure nitrogen inside the tire as the oxygen leaks out. You would probably need a new tire from wear before you see any benefit.