Originally Posted by JCL

JB essentially answered it, but the fault here is my short/cryptic post.

To explain the physics, as I said in my post, air expands when heated just like nitrogen does. They are essentially equal in that respect. However, moist air (which includes water vapour) expands much more, comparatively speaking. It is also the moisture that causes the corrosion problems with the rim.

Racers use nitrogen, and remember that they can get tires much hotter than street use. This isn't because the nitrogen doesn't raise the pressure compared to air but rather that it does so in a much more predictable manner. Racers will measure hot pressures (they have to) and wet air will produce much more variance in hot pressures than either dry air or nitrogen. In those cases, nitrogen is just easier to use. For those of us not measuring hot pressures and bleeding tires down in between laps, but rather driving on the street, using nitrogen is nothing more than a way to get a green valve stem cap.