The tires can be purged and refilled with nitrogen if you can wait 15 minutes, nobody needs to die. If the customer did not request nitrogen, the tires are topped off with compressed air due to time constraints.
That is the reason we fill tires with filter air, the mystery has been solved.
Just so everyone understands: Tires are a bit porous. Gases leak out. Gases leak in. Both of these are controlled by the Law of Partial Pressure of Gases, which states that if there is a differential in the partial pressures, a gas will try to flow from the high pressure to the low pressure.
For example, atmospheric air pressure is 14.7 psi. It is about 21% oxygen, which means its partial pressure is 3.1 psi. A tire filled with 100% nitrogen will gradually leak oxygen INTO the tire until it reaches a partial pressure of 3.1 psi. This happens regardless of what pressure the nitrogen started at.
And this is the real reason some folks sing the praises of nitrogen. That oxygen leaking into the tire prevents much of the pressure loss regular tires (slowly) experience. CR ran a test a while back, found nitrogen tires lost slightly less pressure over a year. Not enough to worry about.
And any concerns over ‘compatibility’ between air and nitrogen are nuts (for passenger vehicles).
To reduce unsprung weight, I use helium. If I want to burn up the track, I use hydrogen
No need to tell me helium&hydrogen molecules are smaller and will leak out faster.
I don’t think so… Maybe if the tire pressure is less than the ambient atmospheric air pressure, then sure, but not at a rate that could be measured without a Gas Forensics Detectors (standard detectors do not detect Nitrogen…) over an extended period of time…
Now, if you want to carry this on into the absurd. If a tire is inflated with atmospheric air which is composed of approximately just about 80% Nitrogen, about 20% Oxygen, and 1% other stuff 
. Then since the molecule for Oxygen is so much small than the molecule for Nitrogen, then it only stands to reason that Oxygen is more likely to leak out verses the Nitrogen…
Conclusion: If you over fill your tires and wait an obscene amount of time, almost all of the oxygen will leak out leaving almost all the air in the tire Nitrogen… 
And 80% of normal air is NITROGEN.
Sorry, but one of the reasons I made that post is for folks who didn’t know so they could to learn a few things.
In this case, it’s that oxygen will leak INTO a nitrogen filled tire regardless of the pressure of the nitrogen. This effect was demonstrated by a paper delivered to Tire Society, Sept 25, 2007 by Don Amos – Study of Oxygen and Nitrogen Permeation through Tires with Monthly Top-off. Unfortunately, this paper was never published.
I do my own oil changes and maintenance along with general repairs. I have a filter on my air compressor. I dont need to check on the policy for either.
I guess you missed the posting I made just previous to your posting…
The reason it was not published after a peer review is… 
It violates Bernoulli’s principle of Fluid Dynamics (in this case, Air is treated as a fluid…) and the Key aspects of the phenomenon: Pressure Gradient Force: This is the force that drives air to move from high pressure toward lower pressure.
Air, like any gas, moves from areas of higher pressure to areas of lower pressure. This is due to the molecules in high pressure areas being more densely packed and moving more rapidly, which causes them to spread out towards areas of lower pressure. This principle is fundamental to understanding weather patterns, wind currents, and the operation of many mechanical devices (like tires… )
Remember, air is not a gas. It is a mixture of gases, and according to the Law of Partial Pressure of Gases, you can treat each component as though it was a separate gas
.
And how would a tire do that?
@CapriRacer is correct. Learned about this in ChemE. Also, the size difference between the two gasses is minor: nitrogen is .364 nm, oxygen is .346 nm.
The oxygen diffuses (not flows) into the tire to equalize partial pressure. Kind of like how pure water diffuses across a semipermeable membrane, even though the pressure is higher on the sugar solution side of the membrane:
I think that all those chem classes and lab experiments are interesting but not applicable in this instance; none of which deals with Inequitable Pressures and only apply to the transfer of gasses at similar atmospheres… as your own example demonstrates…
I remember when “Professor Proton” dealt with this in a similar way when he demonstrated why the Hydrogen molecule (H2) which is only 0.289 nm does not infiltrate the tire…
Don’t forget that “Tire Porosity” refers to the slight, natural permeability of tire sidewalls, which allows small amounts of air to Escape over time, causing a gradual loss of pressure.
Nope. A nitrogen-filled tire loses nitrogen as the partial pressures attempt to equalize, with oxygen move the opposite direction. More nitrogen than oxygen moves (bigger partial pressure delta p) but both move. And the osmosis experiment demonstrates the concept. A rubber tire acts like a semipermeable membrane.
“Professor Proton” says, "A semipermeable membrane does not work the same way when the pressure is greater on one side; rather, the pressure difference acts as an additional force that can speed up, slow down, or reverse the natural flow of solvent.
While a membrane always permits only certain molecules (like water) to pass, the net movement of those molecules is determined by a combination of concentration differences (osmotic pressure) and physical, hydrostatic pressure differences."
Look, we apparently are never going to agree, so let’s agree to disagree…
Does anyone have a link to that Car and Driver study? If I recall, the difference in air pressure loss between a tire filled with ambient air and one filled with pure nitrogen was something like 1.5 psi over a year. That is insignificant, no one could ever feel that difference in a daily driver car.
With an absolute pressure of 49.7 psi (35 + 14.7), oxygen both leaking into and out of the tire, how many decades would it take for the oxygen level to return to 20%?
Life span of the original tire inflation is only 2 to 3 years; tires get patched or replaced, pressure is added every few months. I don’t believe oxygen leaking into the tire makes a significant difference.
That test was flawed. They mounted tires and stored them in a warehouse for a year. Reported a loss of 1.5 to 3 psi after 12 months. In the real world we see a loss of 8 to 10 psi after 12 months.
They might have had different results if the tires were outside and exposed to weather related heat cycles or in use on vehicles.
I inflated customer’s spare tires during 3 year/30K service; compact spare tires are usually down to 20 psi (from 60 psi).
Maybe use nitrogen in just the least often used tire - the spare.
I don’t know, all this discussion about nitrogen vs regular air in tires is making me f a r t .
Bob Newhart is Mr Wizard?
