Originally Posted by SE Steve
snip. From what I have been told is that nitrogen molecules are larger than oxygen molecules. So over time the nitrogen filed tires will lose less air pressure from leaking through the rubber layers
Air = ~80% N2 and ~20% O2, N is 7 on the periodic table and O is 8, add some trace elements (H2O and Ar) and I think you will find the average molecule size is larger for 'air' - for whatever that is worth.
Pure N2 and O2 are both as close as to each other and to an 'ideal' gas that their PV to T relationship is virtually the same.
Pressure changes for N2 vs 'air' filled tyres are due to the water in the air evaporating (=gas), condensing (=water), freezing (=ice) - effectively taking some of the 'gas' out of the tyre.
'Air' filled tyres don't leak any more (or less) than pure N2 filled tyre - unbelievable how there is so much mythology around N2 in tyres!