Performance car enthusiasts and service providers claim nitrogen-filled tires offer benefits like extended service life and lower maintenance. The trouble is that once you pay to have your tires filled with pure nitrogen, you have to top them off with nitrogen in the future. Or do you? Can you put regular air in nitrogen-filled tires?
If your tires have been filled with pure nitrogen, it is perfectly acceptable to top them off with normal air. Furthermore, filling the tires of a passenger car with pure nitrogen most likely does not have significant benefits compared to filling tires with normal air.
This article will explore the rationale behind filling passenger car tires with pure nitrogen, debunk the supposed benefits, and explain why you can add normal air to tires previously inflated with nitrogen. But first, here is a little science.
Nitrogen is the seventh element on the periodic table. In their most common form, nitrogen atoms consist of seven protons, seven neutrons, and seven electrons. The molecule dinitrogen (N2) makes up an average of 78% of our atmosphere at sea level, with dioxygen (O2) making up the other 21%.
Nitrogen was first separated from air by Dr. Daniel Rutherford, a Scottish physician, in 1772. Rutherford did not recognize it as a new chemical element, referring to it as “noxious air,” and noted it seemed “devoid of life” as it could not support a flame.
It was not recognized as a separate chemical element until the last decade of the 18th century.
Nitrogen has been used to fill jet aircraft tires since the 1950s. In 1992, pure nitrogen was used to inflate tires used in extreme motorsports like top-fuel dragster racing and Formula One.
Nitrogen tire inflation was first introduced commercially in 1999 from a chain of British superstores, and by 2007 the service was available worldwide.
As of summer 2021, there are no commercially available tires that specifically require nitrogen inflation. Nitrogen tire inflation is marketed to the owners of expensive performance cars like Ferraris and the Nissan GT-R and motorsport enthusiasts who wish to emulate the spectator sport.
While filling tires with standard air is generally very simple and free of cost, filling tires with nitrogen is most complicated and costs money.
The most common method begins by sucking the air out of a tire with a vacuum pump down to an internal pressure of 3 psi (20.7 kPa) before the tire is reinflated with nitrogen either produced on-site or taken from commercially available canisters. A green valve cover is used to mark tires filled with nitrogen, which has become a fashion statement in the performance car community.
For brand new tires, nitrogen inflation can cost between $70 and $180 per tire. The cost for filling existing tires with nitrogen the first time is a more “reasonable” $30 to $50 per tire, and it will cost between $5 and $7 per tire to top them off. As a reminder, refilling tires with regular air is generally free.
The cost of nitrogen tire inflation could be excused if the benefits outweighed it.
Commercial nitrogen tire inflation providers and performance car enthusiasts claim several benefits compared to filling tires with normal air:
- Slower pressure loss from leaks
- Lower tire maintenance costs
- Slower deterioration of tire rubber and pressure sensors due to the lack of oxygen and moisture inside the tire
- More consistent pressure during temperature fluctuations
In reality, these benefits are either much less significant or do not exist at all.
The reported benefits of filling tires with nitrogen do not stand up to a critical science-based evaluation.
As mentioned above, the air at ground level contains an average mix of 78% nitrogen (N2) and 21% oxygen (O2). The remaining 1% primarily consists of the non-reactive noble gases argon and xenon, water vapor (H2O), and trace amounts of carbon monoxide (CO) and carbon dioxide (CO2).
Regular air at ground level is already nearly 80% nitrogen. Depending on humidity levels, the percentage of water vapor by volume may be as high as 4.4%.
When tires are filled with nitrogen, some normal air is always left inside the tire. When pure nitrogen is pumped in, the resulting internal gas mix will be between 90% and 93% nitrogen and a maximum of 9% oxygen.
According to performance car enthusiasts, the marginally larger molecules of molecular nitrogen do not leak through the tire as readily as molecular oxygen.
Studies conducted by the United States National Highway Traffic Safety Administration (NHTSA) confirmed this in a laboratory setting, at least to a point.
In static laboratory tests, where tires were inflated with several gas mixes and left to sit:
- Tires inflated with air lost an average of 2.13% of their internal pressure per month.
- Tires inflated with between 95% and 99% nitrogen lost an average of 1.39% per month.
- Nitrogen-filled tires performed a whopping 0.74% better than tires filled with regular air.
Consumer Reports conducted a similar test under outdoor conditions. Their experiment conducted over an entire year found an average pressure loss of 3.5% for air-filled tires and 2.2% in nitrogen-filled tires, with nitrogen-filled tires performing 1.3% better.
In terms of time between fillings, the advantage held by nitrogen-filled tires does not amount to even a single trip of more than one hour.
Proponents of filling tires with nitrogen claim the practice dramatically reduces the tires’ maintenance requirements, even making tires “maintenance-free.” They cite lower internal concentrations of oxygen and water vapor, which reduced the rate of deterioration from chemical interactions with the rubber.
This argument does not hold any air.
The aforementioned NHTSA study determined that chemical deterioration does not play a significant role in the failure of tires. Instead, tires primarily fail due to external wear and stress and blunt force or piercing damage from impacts with road debris.
Chemical deterioration will only play a role in tire failure if the tire is left unused for an extended period.
The NHTSA recommends that tires be inspected by a trained professional at least twice a year regardless of what gas is used to fill them. People who inflate their tires with nitrogen are likely to have them inspected more often simply because trained technicians usually perform the refill.
Water could damage a tire pressure sensor if the sensor were submerged in water. Compressed air from commercial compressors is almost dehydrated. Modern air compressors use moisture separators to remove water from the pressurized air piped into the tire. The air inside the tire will contain less than 0.05% water vapor.
Tire pressure sensors are far more likely to be damaged by road debris or a technician using the wrong tool to remove it.
The volume of a gas varies depending on its temperature. Cool gas contracts and warm gas expands. The volume of pure nitrogen gas is less sensitive to temperature fluctuations than regular air.
That is why jet aircraft tires have been filled with nitrogen since the 1950s. It is also one benefit claimed by believers in filling car tires with nitrogen. The more constant internal pressure puts less stress on the tire, which increases the tire’s service life.
Jet aircraft experience a much wider temperature range than cars do. Jets also traverse that temperature range several times during a single flight.
When cars travel, the temperature at their origin is generally about the same as at their destination. And unless the tire is punctured, the tire pressure will also remain about the same.
While nitrogen’s volume is less sensitive to temperature changes, it does not matter for a passenger car.
Commercially available nitrogen tire inflation is likely nothing more than an expensive gimmick. Under the conditions passenger cars are operated, there is no real difference in either performance or longevity between a tire filled with air or pure nitrogen. And therefore, it is perfectly acceptable and frankly advisable to refill nitrogen-filled tires with normal air.
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