After looking around a bit, I discovered that I had not posted all the very good reasons not to bother with nitrogen in your tires, but I did tell several people to look for it. I apologize for that oversight. If you would like to read a thread where I did some of the research and posted it, it is here at MyE28.com. For those who don't wish to read all of our E28-style witty repartee (It does explain where some of my 'attitude' here comes from.), here is a summary: 1) Prevention of oxidation: Some suppliers and users claim that the absence of oxygen in the tire inhibits oxidation and hardening of the rubber. While that is true, it only addresses the inside of the tire carcass, not the outside. Tires oxidize and age primarily from the outside, not the inside. If they oxidize inside, they use up the 21% oxygen in there and it is not replenished until you pump in more air (which, unless the tire was totally empty, will leave much less than 21% oxygen in the new atmosphere inside). The outside is constantly bathed in a 21% oxygen atmosphere, not to mention lots of other ugly stuff (sunlight, ozone, etc.) that will make them harden and crack. Tires degrade from the outside, not the inside. The existence of pumped-up 30-year-old TRX (or 40-year-old XZX!) spare tires is proof of that. So as an oxidation preventative, the effect is real, but inconsequential. 2) Stability of pressure: A gas is a gas is a gas. Boyle's and Charles's Gas Laws are universally accepted as a physical/chemical fact. No gas, regardless of its atomic or molecular weight, behaves any differently inside a tire. While the mass might vary by a barely measurable amount (some gases are heavier than others), the pressure does not. For those who seriously care about weight savings, nitrogen's molecular weight is 28 amu and oxygen's is 32. However, given that air is 78% nitrogen in the first place, the average molecular weight of air is only about 28.5 amu - effectively the same as pure nitrogen. If somebody with a better engineering background than I have would like to calculate the interior volume of a typical mounted tire, I'll be happy to calculate the mass differences for a few selected gases. 3) Dry gas: Another claim is that nitrogen, as a dry gas, makes the pressure more stable because there is no water vapor in it. This only matters when the water changes states. Frozen or liquid water (doesn't matter which it is) in the tire at normal pressures must vaporize to increase the pressure. That is only going to happen above 100ºC - but wait. That's only at normal atmospheric pressure at sea level! Tires are at least twice that pressure! So the boiling point of that water increases dramatically. Even if you could manage to get your tires hot enough to boil any water in there, it is likely to be an inconsequential amount affecting the pressure very little. Besides, there's a very simple solution to this: dry air. Anybody can put a dry air filter on a compressor line for about $10. That's a lot better than 600 times that for a dry nitrogen generator. 4) Less migration: Nitrogen is a smaller atom than oxygen. That, too, is true. So nitrogen suppliers would have you believe that oxygen leaks out of small holes that the nitrogen can't get through. Oxygen's covalent radius (size, when bonded to another atom of oxygen) is 73 picometers. Nitrogen's is 75 pm. BFD. When you recall that a molecule of either nitrogen or oxygen is somewhat dumbell-shaped (consisting of two of those covalent radii) and that gas molecules simply bounce around anyway, any 74 pm-sized holes in the tire would block most oxygen molecules as well. If an air-filled tire lost all its oxygen, that would be only 21% of its fill. Refilling it with air would leave it at about 4.4% oxygen (21% of the 21% of gas replaced), so if you have a bunch of pesky 74 pm holes letting all your oxygen out, two air refills will get you a fairly pure nitrogen filled tire (less than 1% oxygen). Tires lose air because they leak gases, not a gas. 5) Nitrogen is inert: Pure unadulterated BS. Nitrogen is one of the most plentiful active elements we know. Life would be impossible without it. Ever heard of 'amino acids'? The 'amine' is hydrogen and nitrogen. Ever heard of 'nitrous oxide'? Nitrogen & oxygen. Either a 'shot' of horsepower or an emissions headache (NOX). Plants cannot survive without nitrogen. High explosives are largely nitrogen compounds - like the ones that blew up the Murrah Building in Oklahoma City. The only thing nitrogen isn't is flammable ... or a lachrymator. 6) I'm just now seeing claims that nitrogen somehow keeps the tires cooler. I'm at a complete loss to understand how anyone could believe this. I'm definitely waiting for someone to come up with even the slightest piece of evidence that it's true. Nitrogen doesn't conduct or reflect heat any differently than any other gas. The bottom line is that a nitrogen generator costs about $6000. If you've been dumb enough to buy one, you need to convince other people that you were smart in doing so; otherwise, you'll never amortize the thing. So those who try to sell the 'benefits' of nitrogen are very earnest (if not desperately so) in their pitches and they probably really do believe all this made-up stuff. If it's free, go right ahead. Put nitrogen in your tires. It won't hurt a thing. Just don't believe that it makes the slightest measurable difference.