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A friend was informed by his mechanic that his tyres were filled with Nitrogren rather than compressed air. He was skeptical of the claims he heard:

There are major benefits with nitrogen inflation over the traditional compressed air that can significantly increase the life, performance and safety of your tyres.

JAXQuickfit N27 Nitrogen Programme offers you the following benefits:

  • Stable Tyre Pressure
  • Better Grip
  • Safer in all weather conditions
  • Less frequent pressure checks
  • Increased fuel economy
  • Zero internal oxidisation through elimination of moisture
  • Increased tyre life

[...]

Pressure loss is caused by air migration. Nitrogen migrates 3 to 4 times slower than compressed air.

(Claims from an example site.)

I, too, am doubtful. Are any of these claims true?

  • The possible mechanism that occurred to me when first heard these claims was differences in the compressibility and thermal properties of N2 as compared to air, but I haven't actually poured over the tables to find out if that is true. – dmckee May 9 '12 at 15:02
  • no oxidation makes sense because you would be removing oxygen from the environment so nothing could oxidize. – Ryathal May 9 '12 at 15:11
  • @dmckee: So did I, but eventually I remembered my Ideal Gas Laws: PV=NrT. They will both expand with temperature at the same rate. Unless something - water? - is liquifying in the temperature range? – Oddthinking May 9 '12 at 16:05
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    Since air is itself mostly nitrogen (~78%) a factor of 3-4 times difference in migration rate is impossible. Had they been using something big like argon, I might believe it. In fact, if their claims were true, then the "air" in a tire would very quickly become almost pure nitrogen, because those other agile molecules would quickly find their way out. – user3344 May 10 '12 at 10:35
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    @Oddthinking in aviation and racing I believe it's done to prevent risk of fire by removing potentially hot oxygen from the system. Tyres there are also often able to be in- and deflated by the crew from pressure bottles, and bottles of nitrogen are dirt cheap. – jwenting May 16 '13 at 5:50
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Source - British Tyre Manufacturers’ Association (pdf)

Tyre dealers who promote the benefits of nitrogen inflation for passenger car tyres are on the increase. However the British Tyre Manufacturers’ Association cannot endorse the claims being made as they are based on the use and the experience of nitrogen inflation in specialist applications such as aircraft, earthmover and mining machinery, and in motor racing.

On the specific point of tyre pressure:

More worrying from a tyre industry point of view is the claim of reduced pressure loss associated with nitrogen inflation, often with a suggestion that tyre maintenance periods can be extended. The tyre industry recommendation for regular checks (at least monthly and before any long journey) applies regardless of whether the tyre is inflated with nitrogen or air.

The questionable claim of reduced pressure loss is often accompanied by claims of cost savings linked to better fuel economy and improved tread wear performance. Provided tyre inflation pressures are maintained at the levels recommended by the vehicle manufacturer there should be no appreciable difference in performance between tyres inflated with air and those using nitrogen.

This article from the UK Automobile Association addresses the issue of corrosion in tyres.

Corrosion of the tyre from using normal compressed air is unlikely anyway because only the outer tread band of a car tyre contains steel – the amount of moisture reaching it from the inside is minimal.

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There are many sub-claims in the question. I'll focus on only one of them - air loss, compared to nitrogen loss.

This article (Nitrogen loss study) suggests that while nitrogen filled tires did lose a bit less than air filled tires, the difference is not terribly large.

The fact is, air is roughly 78% nitrogen. (I don't think this statement needs any research to back it up.) The remainder of air is mainly oxygen, a bit of CO2, some water vapor, some methane from the cow down the road or the beans I ate for dinner, etc.

The point is, if nitrogen moves so slowly compared to "air" which is mostly nitrogen, then the air inside the tire would quickly become strongly nitrogen enriched, because only the oxygen and other gases leave. Then every time you add air to fill the tire again, the volume inside becomes more and more strongly nitrogen enriched, quickly becoming nearly pure nitrogen after only a few re-fills.

The study I cite in fact suggests that some of this is happening. They state that a 30 psi air filled tire lost on average 3.5 psi, versus 2.2 psi for the nitrogen filled tire. That is, an air filled tire lost 3.5/30 = 11.7% in volume. In that study, a nitrogen filled tire lost 2/30 = 6.7% in volume. This suggests the additional 5% loss in volume must have been almost entirely oxygen. If this is true, then in fact the oxygen is indeed migrating out at a rate that is several times faster than the nitrogen.

The obvious conclusion is that the other stuff that comprises air must be migrating out at a preferentially faster rate. (Unless someone can postulate a mechanism which would enable the oxygen in air to mitigate faster movement of nitrogen too.) The preferentially faster loss of oxygen in the tire would be easily testable. In fact, one could not actually breath the air from inside a tire if it was that significantly oxygen depleted. As I said, this would be easily testable, although I don't know of any tests having been made. In fact, while it is purely anecdotal, I have heard of people inhaling the air from inside a tire, and not quickly succumbing from the lack of oxygen. This suggests a problem with the logic above. Only a test and experimental evidence can resolve the question.

Edit: Some simple algebra suggests that for air to have the greater loss as described in the study, if this is because oxygen is escaping at a faster rate, then the loss rate for oxygen (and argon, etc.) must be 29.4. Thus over a set period of time, 6.7% of the nitrogen will escape, but 29.4% of the oxygen will escape in that same period of time. That extent of oxygen depletion would be easily measured. If this is true of course, then the claim that nitrogen will migrate 3-4 times faster than air is false. While it MAY migrate roughly 4 times faster than pure oxygen, air is not pure oxygen.

Thus, if we are to believe the study (and the conclusions I've drawn from it) then the claim about the loss from a pure nitrogen filled tire has been falsely inflated, as if air was composed of pure oxygen. An inflated claim, strangely like the inflation of a tire. Does this invalidate the other claims? It should cause you to wonder if the other claims were also inflated by a bit, since if one is willing to play with the numbers once, they will surely be willing to do so in other respects.

Re-edit: In an effort to beat this horse completely to death, suppose that a pure nitrogen filled tire did deflate at a rate 3-4 times slower than an air filled tire? What would this imply about the exfiltration rate of the non-nitrogenous components of air? Again, a bit of arithmetic yields the answer. If it is 3x slower, then we have oxygen depleting at a rate of 67.6%, compared to the 6.7% for pure nitrogen over the same period of time. If the difference is 4x, then we would have 98% of the oxygen lost in the same time that 6.7% of the nitrogen migrates out.

Look at this over the long term. Suppose we allow our tire to deflate by 10%, so from 30-27 psi, and then re-inflate it with simple air from the pump. Since most of that air is nitrogen, it is replacing the oxygen we have lost. Clearly this will take only a few cycles before we approach a steady state. That steady state composition of the air in our tire will be approximately 98% nitrogen, and 2% oxygen. So nearly pure nitrogen. Again, the point is, why bother to inflate our tires with pure nitrogen when the simple act of inflation and then re-inflation to replace what is lost will yield essentially pure nitrogen anyway?

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    Poor Argon. Everyone always forgets the third most prevalent atom in the air... – Oddthinking May 10 '12 at 12:31
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    @Oddthinking, well its not forgotten. But being even heavier than nitrogen, argon only will move even more slowly. Anyway, we can pretend a national lab was named after it. ;) – user3344 May 10 '12 at 12:49
  • Edit: I hope that my edits are not construed as original research. Since they are based on the claims in this question, combined with some numbers taken from the only study I could find on the subject, they are simple algebraic computations, no more. – user3344 May 10 '12 at 15:02
  • @woodchips Actually Nitrogen is heavier than Argon, and Oxygen is heavier than Nitrogen! (As N2 and O2 vs Ar) I would assume Oxygen's reactivity would explain the relative rates of loss. – Nick Aug 9 '12 at 16:34
  • @Nick I guess the relative rates of loss are to do with the size of the molecule, not the weight of the atoms. A Nitrogen molecule has a triple bond and is therefore tighter and smaller than an Oxygen molecule, which has a double bond. The molecule's being smaller makes it easier to migrate into a through a container (for a similar reason, it's difficult to contain Hydrogen molecules which are tiny). – ChrisW May 15 '13 at 14:44
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Nitrogen is used in aircraft tyres because the brakes operate at very high temperatures. Thus removing the oxygen from the inflation medium helps prevent oxidation of the tyre and wheel, and in the event of a brake fire does not provide oxygen to feed it.

A fatal accident involving cabin decompression and fire has highlighted another mode of tyre failure in flight where a tyre may fail explosively without any significant prior degradation. A tyre inflated with air and subjected to excessive heating, possibly caused by a dragging brake, can experience a chemical reaction resulting in release of volatile gases. Such a chemical reaction in the presence of the oxygen in the contained air may result in a tyre explosion in a landing gear bay and/or an in-flight fire since it appears that the protection normally afforded by conventional pressure relief devices in the wheel would be incapable of responding adequately to the rapid increases in temperature and gas pressure associated with auto-ignition.

[...] all braked wheels of retractable landing gear units on [a subset of aeroplanes] will be required to have tyres inflated with Nitrogen, or other suitable inert gas, and maintained such as to limit the Oxygen content of the compressed gases to not greater than 5% by volume

However air is approximately 80% nitrogen so the properties of the pure and mixed gases in tyres are similar enough to make no difference for road vehicles.

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http://www.theglobeandmail.com/auto/what-are-the-benefits-of-nitrogen-filled-tires/article697136/

This article is a newspaper Q/A column, where the issue is discussed.

Basically, every benefit claimed in the original question is real, and measurable and completely insignificant for personal automobile use.

TLDR; If you're flying a plane or racing on a track, nitrogen can be beneficial. If you're driving to and from work, you'll probably end up spending more on nitrogen than it will save you.

Also, see: http://news.consumerreports.org/cars/2007/10/tires-nitrogen-.html

My mechanic claims that it stops pitting in alloy rims, but I haven't found any evidence to support that, (Nor have I found any evidence that pitting is a bad thing over the average life of an automobile.)

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