There is quite a bit of confusion in the marketplace about what exactly is the proper purity level for a nitrogen filled tire. It seems the market has moved toward 95% in a passenger tire, and we ship our equipment to meet this demand. But Get Nitrogen recommends >93.4% nitrogen, and Parker agrees. Why? Well, its all about partial pressures, i.e. the pressure which the gas would have if it alone occupied the volume.
We know the atmosphere is composed of 20.9% oxygen and 78.1% nitrogen. So, at atmospheric pressure of 14.7 psia, the partial pressure of oxygen equates to 3.07 psi (.209 * 14.7). Now, consider the 32 psig pressure in the tire, and that same 20.9% oxygen. At 32 psig in the tire, the partial pressure of oxygen now equates to 9.76 psi (.209 * (32 + 14.7)). This increased pressure inside the tire pushes the oxygen out, and oxygen is more than willing to leave. But if you lower the oxygen content in the tire to 6.6% (thus increasing nitrogen to 94.4%), the partial pressure of oxygen is now 3.07 psi (.066 * (32+14.7)). The partial pressure of oxygen inside the tire is balanced with the partial pressure of oxygen in the atmoshpere, and it will remain in the tire longer.
So what happens when you increase the purity above 93.4% in a 32 psig tire? Well, believe it or not, oxgen will begin to leak back into the tire over time. NHTSA said the same at this year's ITEC, as shown on the last slide of this presentation.