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Just seen this thread, so apologies for the delayed response. The chemist in me jut begged for attention:- "Another point is they say the molecule is much larger so less likely to leak through the tyres inner liner or at the bead line" Nitrogen molecule is 155 pm (picometers); oxygen molecule is 152 pm. That's pretty much the same size, so there will be no difference detectable in the gas diffusion rate - whether through the bead or the rubber. What will make the difference is that the pure nitrogen filled tyres should last longer as the oxygen will cause the tyre to perish more quickly from the inside (you can't do anything about the outside!). And, as already noted, the nitrogen is dry, whereas compressed air still has moisture in in. Colin