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Inflating your tires with Nitrogen?

A story about people filling their tires with pure nitrogen, supposedly to get better gas mileage, made me exclaim, out loud, alone in my apartment, "What?!"

I had to read the story. You see, air, which most people use when filling their tires, is about 78% nitrogen. The bulk of the rest is oxygen and water vapor. I can understand the arguments about reduced oxidation on the inside of the tire, but the other arguments seem a bit bogus.

The first of these arguments is that nitrogen molecules are larger than oxygen molecules, and so nitrogen leaks out of the tire slower. This argument is incorrect. The molar volume of nitrogen is 13.54 x 10^-6 m³/mol, while for oxygen it is 17.36 x 10^-6 m³/mol. In other words, if you had enough nitrogen molecules to fill a 10 ft. x 10 ft. x 10 ft. room, you would need 10.86 ft. x 10.86 ft. x 10.86 ft. room for the same number of oxygen molecules. Nitrogen molecules are not bigger than oxygen molecules. Without going into details, the difference between the size of oxygen and nitrogen is neglible given the material (vulcanized rubber) through which they leak. You may also read about kinetic diameters, and for some materials the kinetic diameter of nitrogen is smaller than oxygen. But for some materials, water has a smaller kinetic diameter than hydrogen! I do not know if this is true for vulcanized rubber, but my guess is the difference is neglible. Very specialized materials are produced when making molecular sieves that are based on kinetic radii, and I know of none that rely on vulcanized rubber.

The last argument is that water inside the tire can cause the pressure to be less stable than with pure nitrogen. This might be true, but the higher pressue inside the tire means that less water actually remains as vapor. So while this argument is technically correct, the most one might expect the pressure to fluctuate is 0.5 psi.

The reason most people likely see an immediate improvement when they switch to nitrogen is that they finally have their tires inflated to the proper pressue. In short, the best way to improve your gas mileage via tire inflation is just to make sure you have your tires properly inflated. Also, your tires will likely need to be replaced because of outside treadwear before inside-out oxidation is a problem. The additional cost of nitrogen is not justified by the very limited benefits it provides.