Actually, air is 78% nitrogen.
Nitrogen-filled tires are 100% bullshit; 0% science. This is one of the biggest jokes played on unsuspecting consumers in recent memory.
They have applications in race tires and aircraft tires but it is nothing to do with less "leak-ability." Seriously, do any of these dealers who maintain that the "bigger nitrogen molecules are less likely to leak" even KNOW how big a molecule really is?
I may have slept through most of high school physics but even I know that, saying a larger molecule is less likely to leak through a rim leak or tire hole than a smaller molecule is like saying a 10 big guys are more likely to fill up the Grand Canyon than 10 skinny guys.
An oxygen molecule is only 3% smaller than a nitrogen molecule, and air is only 21% oxygen by volume. Plus, the "nitrogen" they use to fill tires is about 95% nitrogen, so consider the actual difference in molecular size to be infinitesimally small.
Now consider an area the size of the period at the end of this sentence can contain between one million and ten million molecules.
If someone wants extra molecules to help slow leaks, here are some for them ...
Here's some more ...
(Let me know if they run out and I'll give them some more.)
My periods in those sentences above are just as likely to resist slow leaks as the air they sell as "nitrogen" by dealers trying for every profit they can from unknowledgeable consumers.
(Seriously, if nitrogen was less likely to leak through tires, wouldn't one simply have to add air, wait for a few months for all the "smaller" oxygen molecules to leak out, and be at that same 95% nitrogen figure ... for FREE?)
There are NEVER been a single, documented, scientific, double-blind, independent test that has ever proved that nitrogen has the slightest benefit in automotive street tires.
Even Consumer Reports found no statistical difference between 31 pairs of test tires; 1 of each pair filled with air and 1 filled with nitrogen, after ONE YEAR. The leakage difference was so slight that it would have been below the readable accuracy of most consumer tire gauges.