It pretty much comes down to the molecule size of Nitrogen is slightly larger than Air. So the normal leakage of Nitrogen from the inside of the tire to the outside is less than air.
This means that for the normal person that DOES NOT check their tire air pressure on a regular basis, a more consistent and longer tire inflation is achieved.
For many of you that are good about maintaining your cars there doesn't seem to be much difference. At times the Nitrogen argument seems a bit 'gimmicky'. But to each their own. I'll stick with air and just keep up on the maintenance (checking tire air pressure on a regular basis).
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