Originally Posted by booster
All gasses expand at the same rate with temperature, it is if there is moisture in the gas that makes a difference. Nitrogen gas is moisture free, so that is where the gain comes in. Elevation should make no difference per the ideal gas law.
Correct. Dry air (if you could find it) would be almost as good as dry nitrogen.
Many have pointed out that if the "larger molecules" story were significant, the leaking tires would selectively filter by molecule type -- allowing the oxygen to leak while tending to retain nitrogen. Over time, you would end up with mostly nitrogen even if you started with air! Don't count on it.
I agree that if you can get nitrogen for free, there is no downside.