Here's Tire Rack's info on the subject (link):
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=191

On Sat, Oct 17, 2015 at 7:18 PM, John Rodgers <jrodgers113@gmail.com> wrote:
> The only thing the nitrogen does extra is keep the tire from rotting from
> the inside. No oxidizing of the rubber because the nitrogen is inert and
> displaces the oxygen. Other than that, it's a wash.
>
> John
> On Oct 17, 2015 17:11, "Rocket J Squirrel" <camping.elliott@gmail.com>
> wrote:
>
>> "Not sure how he would evacuate the normal atmospheric air form the
>> process [...]"
>>
>> Good question . . . aren't they just pushing nitrogen into a tire that
>> is already at 87% N at atmospheric pressure?
>>
>> Quick, someone do the math: after inflating a tire to, say, 40 psi by
>> adding nitrogen atop regular air at atmospheric pressure, what's our % N?
>>
>> --
>> Jack "Rocket j Squirrel" Elliott
>> 1984 Westfalia, auto trans,
>> Bend, Ore.
>>
>> On 10/17/2015 02:57 PM, Max Wellhouse wrote:
>>
>>> Eric:  I have a friend here in Cedar Falls Iowa that fills his tires
>>> exclusively with nitrogen.  He runs a really small tire mounting and
>>> balancing shop out of his garage and does mobile repairs as well.  He
>>> says that in order to mount tires and fill them with nitrogen, you
>>> have to make sure the bead sealing surface is absolutely pristine, so
>>> there's no chance of micro leaks from pitted sealing surfaces.  I
>>> think he charged like $10/tire to do the fill.  Not sure how he would
>>> evacuate the normal atmospheric air form the process, but I have used
>>> his services on my Vanco II Continentals mounted on Mercedes CLK
>>> forged aluminum rims(mounted on my 90 GL  BTW)  and they haven't lost
>>> 1 psi in like 4 years.  I think Tire Rack offers the service as well.
>>> NASCAR Racers use nitrogen     YMMV
>>>
>>