Originally Posted by Autoputzer

Consumer Reports did a quick test and found that N2 leaks out of tires only about 80% as fast as air. What that means is that the O2 in the air leaks out faster than the N2 in the air. That means that the air in your tires gets "nitrogenier" over time. Water molecules are also smaller than N2 molecules. So, the air in your tires get dryer over time, too.