Nitrogen.
Since the normal air is 78% nitrogen filling a tire with supposedly pure nitrogen garners little benefit for auto tires. This started in th trucking industry where they retread their carcasses for hundreds of thousands of miles. The 18% oxygen or whatever the exact number is deteriorates the tire carcass. I don't keep my Michelins that long for it to make a difference on my auto tire.
The size of the nitrogen molecule makes little difference in migration through the walls of the tires. The lining of tires are designed to minimize diffusion of molecules. CR, I believe it was, tested some tires years back for holding air. They installed on rims and checked pressures. The Michelin X-One was the best at not losing air pressure. Migration in truck tires would be higher because of the pressure run; it's much above 33 pounds for my Michelins. Higher pressure means higher migration through carcass if liner doesn't help and around rim.
Several months ago I checked around the net for info on this. I found LOTS of companies selling nitrogen generators allegedly separating nitrogen out of the air. They claimed a maximum ability at producing pure nitrogen of around 96% for one. That would be maximum, when new, under ideal operating conditions and maintenance. I'm sure they do much, much less at getting pure nitrogen out of air into a tank for the tire dealer to sell to you for your tires. So you're not getting pure nitrogen to begin with. That's unless you see green (or whatever color) hospital style tanks sitting in the work area for them to fill your tires from.
The manner of the advertisements from the companies to the dealer was how much profit could be made per week or month after purchasing various size machines for producing this mixture containing some nitrogen concentrated from normal atmosphere. That's the hook: profit.
Remember selling AOL and MSN to new computer buyers at the computer stores. Something like hundreds off your computer purchase but you agreed to pay AOL $25 per month for 3 years or something. Well, another scam: BEST BUY is selling nitrogen rights
http://www.purigen98.com/BBY-locations.html
The customer is buying rights to keep tires filled with HIGH-purity (not 100%) nitrogen from PuriGen.com company. Apparently the company decided selling machines to tire stores wasn't enough, so now they're branching into selling road service contracts and nitrogen to end customers.
If it's the idea that air is drier, normal compressed air can be run through a drier at most locations. I don't know if the curbside pumps at Speedway have built in driers but I suspect they do. Otherwise their nozzles and hoses would freeze up in the winter temps in much of the country.
So my opinion is don't waste your money.
Buyer beware.
. I got connections, thats why. Of course the brand tires were like weird lol but still they look good and I got my warranty and most importantly no more vibration.
If you want one, check out this thread 
