Airplane tires are filled with nitrogen because they're made of quite different materials than car tires, because they pass through vast temperature extremes within a matter of hours, and because they are enormously more stressed in terms of speeds and weights than are car tires.Race-car tires are filled with nitrogen because it is virtually unaffected by temperature. Travel at 150 or 175 mph on a tire filled with compressed air and the heat that speed generates will soon raise the tire pressure by as much as 15 or 20 psi, and in a serious race car (a Formula One car, a Nascar Cup car) just a one-psi tire-pressure difference can affect handling substantially. In anything from a Toyota Corolla to a Ferrari 599 traveling at Interstate speeds, even Michael Schumacher
wouldn't notice the difference.
The best thing you can do for your tires is to check the inflation pressure once a month, or certainly before any road trip. And the worst thing you can do is fill them with a gas that might persuade you to ignore them.
But boy, do I have a deal for you! For only $50 a tire, I'll inflate yours with helium. It'll make your car lighter, reduce unsprung weight, and give you better gas mileage. Works for the Goodyear Blimp, what's not to like?