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I understand that some places offer a free "tornado" with the nitrogen replacement to give you an even better mileage increase!

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N2 and air are, for all intensive purposes, the same. That is, any heat expansion air has, the N2 will have too. But what N2 doesn't have is moisture and CO2 which do vary vapor pressure greatly with temperature fluctuations a tire will be exposed to. Also, as stated, there is no O2 to contribute to corrosion (or moisture) and the lack of O2 will prevent the rubber from braking down on the inside. For the money these guys are charging for N2 though, man, thats a racquet. I work in the air seperation industry and know what it takes to generate N2.

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According to the ideal gas law (pV = nRT) there is no difference in pressure rise of the different gases given a constant mass and volume.

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I was thinking that myself, but I didn't want to spoil the thread with facts.

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As ealier stated the benefit of nitrogen in the tires is the bigger molecules. They tend to stay inside the tire and tires stay the correct pressure longer. Tire does not have to have a hole in it for air to escape. Air escapes from sound tires over time. Nitrogen escapes more slowly. This is what the higher fuel mileage is attributable to. -Bob

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The question about nitrogen in tires has come up in other lists on the web as well. Let me clarrify a few points.

1. Air is 78% nitrogen, N2, and 21% oxygen, O2. So even if you put air in the tire, it's already 78% nitrogen. Many of the so called nitrogen generators don't produce much more than 90% nitrogen.

2. At relatively low pressures (ie tire pressures) N2, O2 and water vapor will all behave as ideal gases, and follow PV=nRT. Pressure will increase or decrease to the same extent as the temperature increases or decreases regardless of which gas is in the tire. (Even at 300 psi, which is about 20 atm, there is little deviation from ideality.) Therefore the comments about N2 not changing in pressure as the temperature changes are without merit.

3. The rate of effusion (or diffusion) of a gas through a porous membrane depends on the molar mass and to some degree on the molecular diameter. N2 and O2 are almost the same size and N2 is lighter than O2 (28 g/mol vs 32 g/mol) so if either gas were to effuse out of the tire, nitrogen would do it more quickly. Luckily, tires are designed not to be porous membranes.

4. N2 and O2 both have essentially the same specific heat capacity, about 1.0 J/gK, and thermal conductivity, about 0.00026 W/cmK. Water vapor has a specific heat capacity of about 2 J/gK. But remember, water vapor will constitute less than 1% of the air in the tire. So the idea that N2 has different heat handling properties is also without merit.

5. The ozone, O3, in the atmosphere, which is a ground level pollutant, will do a great deal more damage to your tires than the O2 inside the tire. For instance, don't leave a condom out in the air in Los Angeles for a few days. It will develop lots of tiny holes and weaken.

spdracer22 says that dry air is preferably to air with a lot of water vapor. As a tire heats up, the very small amount of H2O present will be in the vapor state which may contribute to the overall pressure very slightly.

Several have suggested that N2 in a high pressure tank is more portable and requires no electricity. That would make sense, particularly for aircraft tires.

I find no reason to believe that N2 is going to produce a "better ride" or "better handling".

The bottom line is that for general passenger car tires or truck tires there is nothing to be gained (other than portability) by using nitrogen rather than air. The biggest gain will be $$$ by the companies that sell nitrogen handling equipment and the tire merchants that appeal to ignorant customers. And who is the biggest loser? Yep, the consumer.

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N2 and air are, for all intensive purposes, the same. That is, any heat expansion air has, the N2 will have too. But what N2 doesn't have is moisture and CO2 which do vary vapor pressure greatly with temperature fluctuations a tire will be exposed to. Also, as stated, there is no O2 to contribute to corrosion (or moisture) and the lack of O2 will prevent the rubber from braking down on the inside. For the money these guys are charging for N2 though, man, thats a racquet. I work in the air seperation industry and know what it takes to generate N2.

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I question the premise that pure nitrogen expands at a different rate than air. According to the ideal gas law (pV = nRT) there is no difference in pressure rise of the different gases given a constant mass and volume.

If you are in the air separation industry, you would be interested in the consumption at the place I used to work at. They used liquid nitrogen to cool copper conductors for high magnetic field magnets. In a year, the experiment uses ~2 million gallons of LN2 at a cost of over $500K.

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So who has the $500k per year account for your LN2?

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According to the ideal gas law (pV = nRT) there is no difference in pressure rise of the different gases given a constant mass and volume.

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oh, oh, I know this one!

6.022 x 10^23

What do I win?

(It's been about 30 years since my college chem class, and you have no idea how little I get to use stuff like this.)

But I'm sure the question everyone wants answered is "what about acetone in the tires?" (I bet you thought I was gonna say ear wax...)

Back to the original subject, you have to admit it's a pretty clever idea to separate consumers from a little extra money on a tire sale: $5 per tire to fill it with nitrogen.

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According to the ideal gas law (pV = nRT) there is no difference in pressure rise of the different gases given a constant mass and volume.

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oh, oh, I know this one!

6.022 x 10^23

What do I win?

(It's been about 30 years since my college chem class, and you have no idea how little I get to use stuff like this.)

But I'm sure the question everyone wants answered is "what about acetone in the tires?" (I bet you thought I was gonna say ear wax...)

Back to the original subject, you have to admit it's a pretty clever idea to separate consumers from a little extra money on a tire sale: $5 per tire to fill it with nitrogen.

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I've been wondering about why NASCAR uses nitrogen if there is little difference in pressure rise with temperature. I wonder if it has to do with the presence of oxygen in the air under pressure and the potential for explosions in an accident? Anyone know?

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From what I understand it's out of convienence. They have pressurized nitrogen readily available and don't need to have a compressor. I'm not sure what they already use the nitrogen for, maybe someone else does?

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I've been wondering about why NASCAR uses nitrogen if there is little difference in pressure rise with temperature. I wonder if it has to do with the presence of oxygen in the air under pressure and the potential for explosions in an accident? Anyone know?

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"Back to the original subject, you have to admit it's a pretty clever idea to separate consumers from a little extra money on a tire sale: $5 per tire to fill it with nitrogen."


I guess they are just no smarter than the average consumer. -Bob

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That still doesn't explain why most turbine aircraft manufacturers and the FAA pretty much mandate "dry nitrogen only" in aircraft tires. They do it primarily because it gives the least pressure variation with temperature change and secondarily because moisture causes corrosion and can freeze at altitude. It has nothing to do with portability. Almost every small shop has an air compressor, while not all of them bother with the expense of a nitrogen tank setup.

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What do I win?

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You win a cold 40oz. bottle of Steel Reserve 211!

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