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Nitrogen or Air?

Published in the August 2011 issue of “Die Porsche Kassette”

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Pedro

Nitrogen or Air in the tires? by Pedro P. Bonilla (GCR PCA)

To obtain additional information about tires, how to fix them, fill them, and more, please visit my website at: www.PedrosGarage.com.


Happy Porsche-ing,








© 2011 Technolab/PedrosGarage.com

If it hasn’t happened to you already, when you purchase and mount a new set of tires on your car, you will likely hear from the tire shop: “Nitrogen-is-better-than-air for your tires” ... and “we offer the nitrogen-fill option at only $$” (generally $5 - $20 per tire).


For many years now, nitrogen has been used to fill aircraft tires, including the Space Shuttle’s and also tires on race cars, but it’s use on street cars is relatively new and somewhat controversial.

The tire shop will tell you that nitrogen in your tires:

•Improves steering

•Improves handling

•Improves braking

•Reduces the chance of tire failure

•Dramatically slows pressure loss from permeation

•Improves fuel economy

•Reduces tire oxidation

•Eliminates interior wheel corrosion

•Reduces running temperatures

•Decreases false alarms and activation of your TPMS (tire pressure monitoring system)


As with most marketing there is a little truth to it but the rest is mostly HOT AIR!

Nitrogen molecules (N2) are larger than oxygen molecules (O2) so therefore, pure nitrogen will permeate the walls of your tires less than oxygen molecules.



But by how much?

Well, a nitrogen molecule measures roughly 300 picometers while an oxygen molecule measures 292 picometers.  That’s only a 2.6% difference in size. One picometer is equal to one trillionth of a meter (1 m / 1,000,000,000,000).

Consumer Reports did a study in 2006 where they measured pressure loss of nitrogen-filled Vs air-filled tires over a one year period.  They took 31 pairs of all season, automotive tires (H and V speed rated).  One tire of each pair was filled to 30 psi with air, the other tire from the pair was filled to 30 psi with nitrogen.  All 31 pairs were then set aside, outdoors for 12 months.


Their conclusion was that nitrogen does reduce tire pressure loss over time, but the reduction is only 1.3 psi. 


Air-filled tires, originally filled to 30 psi lost 3.5 psi over a one year period. 

Nitrogen-filled to the same starting pressure of 30 psi lost 2.2 psi over the same period.

More importantly ALL tires lost pressure, so consumers should check their tire pressures routinely regardless of the gas used.


The air around us, in our atmosphere, what we breathe and what is in most automotive tires is a mixture of gasses.

Air = 78% nitrogen + 20.95% oxygen + 1% other gasses ( 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases and a variable amount of water vapor).

So, already all of the above claims are not true by 78% because going to pure nitrogen will only replace 22% of the tire’s internal volume.


The claims of improved steering, handling and braking, as well as reduced chances of tire failure and improved fuel economy are true for properly inflated tires regardless of the gas used to fill them.


Then there’s the claim regarding the reduction of internal tire oxidation because nitrogen is inert and oxygen is corrosive.  I don’t think any of us have ever replaced a tire that had internal rubber decay because of oxygen corrosion.  Although it is partly true –oxygen is corrosive-, your tires will wear out many times over before oxygen starts to damage the rubber material.  If this claim were true and oxygen causes rubber corrosion that quickly, what about the outside of the tire?  It is exposed to 22% oxygen even if you have pure nitrogen on the inside!


Some people also will tell you that tires filled with nitrogen don’t change pressure as much with heat as those filled with air, or that tires filled with nitrogen run cooler than those with air and that’s why most race cars including F1, Indy and NASCAR all use it.

Again, not true. 

At the temperature (150 – 250ºF) and pressure (25 – 45 psi) thresholds found in racing tires, both air and nitrogen will act as “ideal gasses”, meaning that they will react exactly the same to temperature and pressure.

Oh, I almost forgot.  There are those that claim that since nitrogen is lighter than air you’ll save weight and have better performance.


OK, so let’s analyze this one, I mean, less unsprung weight is good, no?


The weight difference between oxygen and nitrogen is less than 3%, but let’s take the full 3%.

One 255/40/17 summer tire holds approximately 13.88 liters (0.5 cu.ft.) of air at 2 bar (29.4 psi).

One liter of air is roughly equal to 1 gram so there are roughly 13.88 grams of air per tire, 55.52 grams in all 4 tires.  Nitrogen is 3% lighter than oxygen, but there’s only 22% oxygen in air, so the difference in weight from all 4 tires is 0.3664 grams, that’s a whopping one third of a gram!

Here’s the math: 55.52 gr x 0.03 x 0.22 = 0.3664 gr

To put it in perspective, one official ping pong ball weighs 2.7 grams.


So, in essence, filling your tires with nitrogen won’t hurt anything and will provide some minimal benefits.


But, is it worth it?

Absolutely ... if you go to Costo or other shops where it’s a free service with your new tires.

But paying an average of $10.00 per tire is absolutely not worth it.


Furthermore it may give you a false sense of security where you won’t regularly inspect your tires.  Remember, even filled with pure nitrogen your tires will lose pressure over time.


The real reasons why most race teams use nitrogen gas in their tires are:


•   Nitrogen is an inexpensive, non flammable gas that can be transported safely in high-pressure bottles

•    Nitrogen is dry (no water vapor)

•    Racing teams strap nitrogen bottles onto their tents’ legs to hold them down.

•    Nitrogen tanks nearby allows them to fill or adjust tires and to run their pneumatic tools without the need for an air compressor which requires electric power (generator).