i use nitrogen. i have access to a rig that evacuates air out of my tires before i fill it with nothing but nitrogen. it'll vacuum all the air out of the tire, sucking in the sidewalls and collapsing the tire making it look like a dried up prune... all without breaking the bead seal.. then it fills with n2.

common air is about 72% nitrogen, 26% oxygen and 1% of other mixed stuff... (you learn this in scuba class). the benefits of nitrogen in the tire are not hype. all pro race cars use this. why?

- the humidity in nitrogen is less then 1%. humidity in regular air can grow over time if the tanks on air compressors aren't serviced and drained regularly. the older a tank/compressor, the more chances of water are in the tank and henceforth filled into your tire.

- high water content is bad for the tire and therefore seep through and leak air.

- the reason why they are important for serious track and heavy duty use is once they heat up, the air pressure does NOT expand as much as regular air due to the low levels of humidity. On a hot day with surface temps above 120 degrees, my friends that use air in their tires experienced pressure increases as high as 18 psi. I on the other hand only saw 2 psi increase all day long. my friends have to constantly bled their tires and play with pressures all day long while i just drove and had fun...

costco is now moving to filling tires with nitrogen too... so its becoming mainstream finally...

i'm in the middle of building my own nitrogen rig for my tires since i go through so much tires...