nitrogen tyres.

[Farjo]

Perhaps tyres will end up with almost 100% nitrogen after a while and here's my theory. Buy a new tyre which is filled with normal air (78% nitrogen, 21% oxygen). Some of the oxygen permeates through the tyre so you re-inflate it with air. Assuming the tyre pressure went down by 10% the tyre is now filled with the original 78% Nitrogen and 21% Oxygen, minus the lost 10% oxygen, plus the replaced 7.8% Nitrogen and 2.1% Oxygen = 85.8% Nitrogen and 13.1% Oxygen.

Repeat several times and the proportion of Nitrogen will steadily rise, not to 100% but presumably into the high-90s%.