Waiter, there's water in my air
dry air is 78% nitrogen already - so the difference is not in the "air" part, it's in the "dry" part.
it's the water vapor in the air that makes the diff, kids.
- water vapor will happily condence and re-vaporize inside of your tire - and thus you'll get a range of tire pressures
- ever notice that if you blast a bit of air out of an air hose that it feels wet? well, when you expand a gas, it cools (that's pV=nRT), so you get extra liquid water in your tires each time tht you fill them from the hose. and then you go out and zoom around...and it vaporizes, or condences...
of course, the actual difference is pretty small, and maybe only valentino can really feel it.