I’m new here, so i don’t really no what I’m doing, but I noticed that in this column: http://www.straightdope.com/columns/070216.html
cecil says nitrogen effuses (exits a small leak) slower than oxygen. But N2 weighs less than O2, so by graham’s law of effusion and common sence, nitrogen should escape the tire faster.
oops, i meant nitrogen in the title
Welcome to the Straight Dope Message Board, megas alexandros! Other than the mixed up title to the thread, you did just fine! You posted something important to say, and you included a link to the article under discussion. 
Stick around: someone will be along in the very near future to address your comment, no doubt. Pull of a chair and poke around the other fora; we’re kinda fun if you let us grow on you.
This PDF explains it. Briefly:[ol]
[li]Graham’s Law applies to effusion, not to permeation through a solid (such as rubber).[/li][li]The key factor in permeation is molecular size.[/li][li]O[sub]2[/sub] is smaller than N[sub]2[/sub]. There is no obvious relationship between atomic number or atomic mass and atomic diameter. See this diagram.[/li][/ol]
OK, that makes sence
Actually, the relationship is that the radius decreases to the right due to increased charge on the nucleus. Also, since both N[sub]2[/sub] and O[sub]2[/sub] are diatomic, their atomic radius may not be the size determining factor. O[sub]2[/sub] has effectively (minus a small amount of anti-bonding from the HOMO) a double bond, while N[sub]2[/sub] has a triple bond. Typically, a triple bond will be shorter than a double bond so it might be expected that N[sub]2[/sub] is smaller.
I don’t know what the actual numbers are so O[sub]2[/sub] may be smaller, but referring to the atomic radius for the size of a molecule is wrong.
It’s a lot better for the profit margins of the people selling the nitrogen.
But there’s no evidence to show it’s better for the buyers.
Yes and no. In this particular instance, comparing the permeation of N[sub]2[/sub] and O[sub]2[/sub] through rubber, “[t]he differences observed are essentially solely due to the slight differences in the size of O2 relative to N2.” (from your pdf) In general, a key factor is the molecular size, but in addition the solubility of the gas in the material must be considered: “[D]ifferent types of gases will permeate each material faster or slower depending on the relative sizes of the gas molecules, as well as on how soluble the gas is in the solid material.” (also from your pdf; my italics]