A friend sent me this years ago.
What do we know or can we reasonably infer about nitrogen inflation?
1. According to NHTSA, nitrogen filled tires (N) lose 1.4% of their pressure per month on average compared to 1.8% for air filled tires (A). Using those values, the difference between a N and an A after a year is 1.3 psi (the same number arrived at by the CR study) if neither tire is topped-off during the year. Obviously, frequent top-offs are required by both N and A. As far as pressure retention is concerned, this proves that there is a slight, but relatively insignificant advantage to N.
2. According to both NHTSA and Ford(Baldwin), there is a net inward permeation of oxygen in N. Significant permeation occurs within 90 days in static tests. According to NHTSA, both in the lab and on the road, oxygen permeates outward faster than nitrogen in A leading to a significant enrichment of nitrogen in A. This inward and outward permeation will continue until chemical equilibrium is achieved. It is reasonable to assume that such equilibrium will occur between 3 months and a year such that the gas in a N and an A will have near identical chemical compositions by that time. So, once equilibrium concentrations are established (within 1 year) there is no difference between N and A from then until the tire is replaced.
3. Because water vapor permeability is 80 times that of oxygen, water vapor concentration will also equilibrate in a manner similar to oxygen in 2, above.
4. The thermophysical properties of dry air and nitrogen are nearly identical. In the operating envelope of the average passenger car tire, both gases may be treated as ideal gases. Water vapor has a much higher heat capacity, but it too may be treated as an ideal gas. Further, it is a tiny component of the gas mixture. Within the operating regime of the average passenger car tire, the water vapor does not significantly effect tire temperature, pressure, or any other parameter. So, there is no physical reason that the two gases should influence either the handling or ride comfort of a vehicle. If, someone's calibrated rear end were somehow sensitive enough to detect a difference, then from 2 and 3, above, that difference should disappear sometime within a year.
5. According to NHTSA, there is no difference in rolling resistance between N and A. This strongly implies that there should be no expected difference in gas mileage between N and A. The conclusion of 100% of the existing literature is that if tire pressure is maintained, there is no difference in mileage between N and A. Tire pressure is the only determinant, all other things being equal.
6. Ford(Baldwin) and NHTSA have shown that a 50/50 mix of nitrogen and oxygen baked in a tire at 65C for 5 - 12 weeks (with O2 replaced as permeation occurs) will produce oxidation damage to a tire equal to 6 - 8 years in Phoenix, AZ or 8 - 12 years in the northern part of the Country. The NHTSA has reported that the average passenger car tire is replaced due to wear at 3.6 years. What does this mean? It means that oxygen does indeed cause damage to tires if given enough time and temperature. It also means that tires wear out long before oxidation becomes a significant factor. Average passenger car tire life is clearly independent of oxygen content inside the tire and the slow oxidation reaction that occurs.
7. Exterior tire sidewall damage is due to reaction with ozone, UV irradiation, abrasion due to a variety of road hazards, and to some degree inward oxygen permeation. Tread wear is due entirely to friction contact with the road surface and varies with driving style, road surface (friction coefficient), temperature, tire pressure, wheel alignment, suspension repair & tune, wheel balance, and other factors. The interior fill gas has no bearing on exterior sidewall life or tread wear other than the pressure it exerts.
8. New tire materials and manufacturing processes have been developed over the past decade of so that even further minimize any differences that one might expect between A and N.
The overwhelming conclusion to be drawn from the above, is not only is there no economic reason to fill average passenger car tires with nitrogen, but there is no compelling reason to fill average passenger car tires with nitrogen even if you can get it done free.
