well, yes, you have to replenish the missing O2, no? i wonder if the missing oxigen actually reacts with rubber and not simply "diffuses out". this reminds me old oil bottles that collapse with decreased air volume.
If you do the calcs the O2 does not immediately leak out....it takes several years. But the evidence does not seem to support that N2 filled tires last longer. So the main benefit of N2 is slightly slower leakage rate, which can be compensated for by checking tire pressure. We are so fortunate air is 79% N2, otherwise we'd probably have fill tires weekly.
Several months or at least several weeks to permeate out enough to significantly affect tire pressure. It will never escape beyond the point that partial pressure of the oxygen inside the tire equals the ambient partial pressure. Getting near that condition might take several years.
Having real-time tpms readouts, a cold tire to a warmed up tire running at freeway speeds is at most 3psi greater using regular air, which is not enough for me to justify using nitrogen.
I run that 80 -20 blend in my tires that's what I tell then when I go tot he tire store If you get it mixed like that it runs better in the tires
I decided to research CO2 a little further (for my bike tires). Got myself a CO2 pump from Amazon. Apparently however CO2 leaks very fast thru rubber, if I understand, due to solubility of CO2 in the rubber. In other words, molecular size is apparently not the only variable. But interesting, you can get a special gas called STAYFILL which is designed to not leak. CLICK HERE FOR STAYFILL FAQ Its a very heavy gas apparently CF3I fluoro-iodo-carbon. They claim it's OK for the ozone layer. I will try this for my bike. Believe my CO2 pump will take the cartridges. Bottom line however thank goodness nitrogen is such a slow leaker through rubber. Most other gases leak faster (except if you get into more exotic stuff).