Nitrogen denser than Oxygen? I dont think so. Atomic weight of N is
14, Atomic weight of O is 16. N forms N2 and O forms O2 for respective
weights of 24 and 32. From ideal gas laws PV=nRT, n/V=RT/P. n is the
same in tires filled with air, Oxygen (O2)or N2 thus Oxygen is denser
than Nitrogen.
You could fill the tires with CO2 which would help extinguish tire
fires and would leak out of the tires slower than air. For that
matter, when putting the tire on the rim, you could put a block of dry
ice in it and it would self inflate.
Maybe inflate the tires with freon, which would leak very slowly,
somehow connect it ot your air conditioner and............
Or Radon, when you stopped detecting its radiation your tires would be
low.
For the serious spender, try Xenon which is about $700 for one of those
tiny lecture bottles.
If you are afraid of static electricity, use SF6 (Sulfur HexaFluoride)
to soak up electrons.
Do I have too much time on my hands?