你想當然！

"考虑到空气中氮气占78%，氧气21%,这种改善有限。而且气体膨胀是差不多的。"

The difference is not 78% vs 100% nitrogen or 气体膨胀是差不多的.  It's water in the air.  What about change from liquid water to water vapor?

My personal experience also tells me that nitrogen is much harder top eacape from tires, which I didn't believe before.  I kept checking my tire pressures for years and there is no significant changes at all and I stopped checking it since I have low tire pressure warning feathure on my car.