Purity continues to be the most misunderstood, misapplied issue in nitrogen inflation. I recently heard this week that anything under 99% is unacceptable. Hogwash. The majority of benefits of nitrogen tire inflation (better gas mileage, safety, tire life, etc.) are derived from better pressure retention. As you can see from the Dr. John Daws Nitrogen Inflation Study, the proper purity for passenger tires (35 psig) is approximately 93.5%, and this is the purity that maximizes pressure retention. As tire pressure increases, so does the required purity (as high as 98% for a 100 psig truck tire).