And you asked about laughing gas. That's nitrous oxide, used by some dentists as a mild anesthetic. It is in no way related to the nitrogen used in those tires -- nor to the nitrogen in the atmosphere.

For almost 40 years, I have been using a special blend of over 70% nitrogen, with about 20% oxygen, and several other gases, and I have been very happy with the results. It is available at many gas stations, and goes under the name of "AIR"!

Air at sea level is 21% oxygen, 78% nitrogen and 1% argon. There's a very little bit of other stuff in there, too.

I suppose that over time, oxygen might react with the rubber, but it should not be much. I believe that anyone using a nitrogen charge, like aircraft or race cars, does it to reduce burn potential as mentioned above. I see no particular reason for most people to use nitrogen only.

Nitrogen molecules are larger than o2 molecules so the tire will not dissipate (spelling?)the gas as quickly therefore, keeping the tires at the proper psi longer. BUT, the nitrogen will leak anyway and when you need to refill you must return to that dealer. Do you have time for that? I don't. Nascar uses nitrogen for good reasons but nobody on the street drives down rt 66 @ 178mph. Also o2 or air has been used ever since inflatable tires were invented so I believe they are good enough.

Race tires are also much thinner, use a less dense compound, and (because they're thinner and softer...and the speeds those guys travel) are far more sensitive to changes in air temperature than passenger car tires. Race tires don't have to go 50,000 miles...or go over potholes. They can be thin and soft.

Oh, and nitrogen won't feed a fire in the pits like oxygen would. That's why sanctioning bodies prohibit it.

"Nitrogen molecules are larger than o2 molecules..."

That is true, but just barely. The difference is so small that I doubt there is a measurable difference in diffusion through the tire or past mating surfaces. The covalent radius of nitrogen is 75 picometers; oxygen has a covalent radius of 73 picometers. Just double both values to get the length of the diatomic molecule. The difference is less than 3%.

Don't you need to quadruple (not double) the radius to know the size of the molecule? Doubling it will tell you the distance between the two nuclei (i.e., the bond length), quadrupling it will tell you the size from the outer edge of one side to the outer edge of the other. Regardless, the difference between O2 (73 pm * 4 = 292 pm) and N2 (75 pm * 4 = 300 pm), is less than 3%. That said, a few months back Consumer Reports did demonstrate that tires filled with N2 held their pressure slightly better than those filled with air.

Oh leave her alone. She thinks she's saving the planet like a former VP does. That's worth it to her to minimize her damage to the earth while here. So its a win win. A hundred years from now no one will know she was here, and the guy at the tire shop got his $30. Hee hee hee.

Here is the brain twister. If nitrogen does not increase in pressure much as tire temperature increase, then nitrogen filed tires may decrease gas mileage. Here is why, if two sets of tires with equal pressure, one with nitrogen, and the other with air are both driven to operating temperature then the one with air would have a higher pressure, about one pound for every ten degrees rise in tire temperature, and therefor the tires with air will get the better gas mileage. The tire with nitrogen will always run close to cold tire inflation pressure. Don't know if tire, and car manufacture have different recommended inflation pressure using nitrogen.

Both tire, and car manufactures set recommended tire inflation pressure based on cold tire pressure with the increase in pressure when the tire gets to operating pressure factored in.

I thought the whole point of nitrogen-filled tires was that the pure gas is DRY. Its chemical properties have little or no effect, but the lack of water can have a noticeable effect on minimizing pressure change due to temperature change. You should be able to get the same positive effect by using a very good dryer on your air compressor.

"I thought the whole point of nitrogen-filled tires was that the pure gas is DRY. Its chemical properties have little or no effect, but the lack of water can have a noticeable effect on minimizing pressure change due to temperature change. You should be able to get the same positive effect by using a very good dryer on your air compressor."

I believe that is correct, both dry N2 and O2 will act about the same at these relatively low pressures and temperatures (close to an ideal gas), water/water vapor is the only real variable that is eliminated by using pure N2.

Perhaps if it were Helium, the vehicle would be a bit lighter, and that would translate into some mileage increase. Probably not enough. What if you filled the passenger, and trunk areas with helium? Compressed helium! Now were talking (albeit in a higher octave). Hmmm. must retire to the porcelain receptacle to further contemplate.

This is my first time on this site and I've read the, so far, 28 replies to this nitrogen question.
The thing is, I'm pretty sure Tom and Ray dealt with this question, maybe a couple of months
ago in their Car Talk newspaper column. I don't recall exactly what they said, but am suprised
no one has so far mentioned it. (I think their bottom line was about what everyone seems to agree
on, that the difference is negligible and the cost a gyp.) As I have only disagreed with what they had to
say twice in all the years I've read them, I tend to trust their "lousy" answers.

Apparently Tom and Ray get calls from time to time on this issue. Here's one of their answers:

http://www...ry/02.html

The quick answer is that it isn't a problem unless you are a race car driver.

They have it (more or less) correct, see my previous post.

Being that Air (you know, the stuff we all breath in and out each and every day with each and every breath and normally fill our tires with) is apporximately 80% Nitrogen, 19% Oxygen and 1% trace gases like CO2, CO, Water Vapor, Argon, etc., I can hardly see how 100% Nitrogen is much better than 80%, epscially since most gases behave, at least thermodynamically in the old PV=nRT fashion, more or less the same (except for the water vapor which would be likely to condense and possibly even freeze to form ice at normal automotive operating temps But that is negligible). So other than possible oxidation of the innner tire walls or the wheel, I can see no advantage to using pure nitrogen instead of air, for a nirmal car at least. As some pointed out, there is a great advantage to suing it in Airplane Tires and possibly even racing car times, but clearl;y none for regualar old cars, even for better fuel economy.

- When buying compressed gas from a supplier, it's not necessarily "dry". So, although it's likely to be drier than compressed air from a gas station, I wouldn't *assume* it to be so (unless I looked at the nitrogen tank and noticed that it was labeled "Bone Dry"). Besides, even if the air were saturated with moisture, I'm not sure it would make a difference to the longevity of the tire (most tires get replaced because they are punctured, wear out, etc., not because they 'dry rot' from the inside).
- For all practical intents and purposes, as temperature increases all gases increase in pressure similarly.
- A few months back Consumer Reports tested nitrogen in car tires and demonstrated that it leaked out slightly more slowly than air. That said, what's the point? Unless the tires were evacuated before filling with the nitrogen, there's plenty of oxygen in there already, and adding air to the tires will add more. BTW, you should check your tires AT LEAST once each month and inflate (or deflate) them according to the standards listed on the sticker inside the driver's door jamb (NOT the pressure listed on the tire!!). Even if your tires never lost any air (which is highly unlikely!), you'll need to vent some pressure for the summer, and add some for the winter. That will end up adding plenty of air (i.e., gases other than nitrogen).
- Your sister should ask for her money back, and the mechanic who rooked her should be publicly exposed as a scam artist.

Air is a mixture of gases, 78% nitrogen and 21% oxygen with traces of water vapor, carbon dioxide, argon, and various other components. Nitrous oxide (laughing gas) is N2O. This guy should have given her that so at least she'd have a giggle or two while forking over 30 bucks for an additional 20% of nitrogen fill which will do nothing ..... BOOOOOOGUSSSSSSS!

There is nothing wrong with nitrogen, what is wrong is paying $30 for it. The main advantage is that it is perfectly dry so there never is any condensation inside your tires to rust or corrode your rims.
I can't, for the life of me, see how nitrogen could possibly decrease your tire's rolling resistance.
Nitrogen is cheap, it basically is a by-product of refining air into pure oxygen. If you didn't have a use for it, they would just have to release it back into the air anyway. The cost of this stuff is basically the cost of compressing it into cylinders and transporting it.

"The cost of this stuff is basically the cost of compressing it into cylinders and transporting it."

The refrigeration cost to separate the gases is a large part of the purchase price, too.