I suspect it has to do with the tire compounds.
Tires on GT3's seem to have softer compounds, which are better for the track.
My $02.
Mr. B

 

Because they come with those sh%ty PS2's which have very weak side walls and need lots of air to stiffen the up!

 

Well, I actually liked the PS2's that were on my C2S. As to the Pirelli's, I'll have to wait until I can get them on the track to see what they're like.
Mr. B

 

Actually on the 997s I have found the PS'2s to be quit adequate. I'm talking about the GT3 two distinctively different cars, I would equate that with Michael Jordon playing basketball with a pair of Wolverines on!

 

Are all of you guys running nitrogen?

 

Just air for me.
Mr. B

 

Nitro! Always.

 

Air only.

 

I see no benefit to nitrogen.

Here's what the experts say:
"Most tires are filled with compressed air, which when dry consists of about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases by volume. Water vapor (humidity) can make up as much as 5 percent of the volume of air under worst-case conditions. Filling your tires with nitrogen mainly does two things: it eliminates moisture, and it replaces skinny oxygen molecules with fat nitrogen molecules, reducing the rate at which compressed gas diffuses through porous tire walls. That means, theoretically at least, that a tire filled with nitrogen retains optimal pressure longer, leading to more uniform tire wear and better gas mileage. The commonly quoted figure is that tires inflated to 32 psi get 3 percent better mileage than at 24 psi. Does nitrogen make any practical difference? You couldn't prove it by me. I found no scientific tests showing that nitrogen-filled tires stayed inflated longer than average under normal conditions. A car-buff buddy was sure it worked but conceded he had only anecdotal evidence that it did.
As for moisture, changes in humidity affect tire performance two ways. First, the density of humid air fluctuates more with temperature than that of dry air, so removing humidity can keep your tire pressure more consistent, especially when the temperature climbs over 100 degrees Fahrenheit. That may be a legitimate concern in Formula One racing, but it's not much of an issue if you're just tooling around town."

Mr. B

 

a lot of competitive auto-x people do that, just to minimize pressure changes on a short track when hoosiers A6 go hot.

regular r-comp tires like NT01 or R888 do not really benefit much from it. dry air work fine.

 

I use air in my racecar too, which runs on the Hoosier "R" compound tires.
Mr. B

 

I read somewhere ( will post if I find it) that air causes the rubber to degrade as to where nitrogen does not. I have been researching this quite a bit just out of curiosity. For a street car, there really is no reason to run nitrogen over dry air.

 

We use nitrogen in racecars for the simple reason that w/o any water in it you get more consistent and reliable changes from cold to hot. I can fill a tire cold to X pressure and know exactly what it will be at in 1/2 a lap. Air will not be as consistent.

The differences in stock cold pressures for different models is mostly attributable to differnces in weights of the vehicles.