This was taken from a local paper that one of Arizona's more known mechanic had to say about the issue.

"
Studies show there is a difference of air pressure of 3 to 4 pounds annually between compressed air and nitrogen. Yes, the air in your tire is 80 percent nitrogen, however, over the last three years of testing of nitrogen-filled tires, using a sophisticated sensor, I have yet to find a nitrogen-filled tire that has more than 95 percent nitrogen. That is because many machines cannot make pure nitrogen. Most will not tell you what percent they make.
Secondly, there is no way to suck the remaining air from a deflated tire to create a 100 percent virgin nitrogen-filled tire. That is exactly why our testing shows the average nitrogen-filled tire has 89 percent nitrogen.
So here is what I say to my 45,000 customers. If you want to change the nitrogen in your tires from 80 percent to maybe 90 percent or less, go ahead and pay $29 to $89 to have a shop fill your tires.
But some will tell you, from that point on, you must go to them for tire inflation service. If you think that is a good idea, why not give the key to your locking gas cap to one station? This is another way for some in our industry to take money from your pocket and put it in theirs and give you nothing in return.
Mark Salem, mechanical expert Phoenix KTAR Car Guy
Salem Boys Auto, Tempe ASE Master Tech, L-1
http://www.paysonroundup.com/news/20...-use-nitrogen/

Think I will keep my money. That is why I own an air compressor. Its easy to check tires once.

At my dealership, it is $149 (+ 13% of course) for a "lifetime" of nitrogen fillups.
Sounds like you guys are getting off cheap at $20/tire.

well i can't say anything that hasn't been said, exsept maybe I've had tires oxidise on the outside before the inside using air. If i could find some place that had a pure nitrogen enviroment to mount and fill my tires I might let them do it.

Nothing personal, but I think your dealership is robbing their customers...

There are those that insist that the "checking" on the outside of the tires results from the catalyst of ultra-violet light acting on ozone(O3) in the air. Inside the tires there is no ultra-violet light that I'm aware of.

same thing happened to me with my tires, but mine are some pretty thick rubber. but what happened was the pressure wasnt where it was supposed to be at. so every morning they were about 10 lbs lighter than they were a few minutes later after driving on em.

Interesting... we've had some fairly cold weather (teens and single digits) and every morning my tires are down exactly 1 psi - and inside of 3 minutes they're up to normal pressure.

I couldn't agree more!