Dear Tom and Ray:

I recently heard about filling tires with nitrogen gas to maintain pressure and lengthen tread life. Since the air we breathe is 78 percent nitrogen, I fail to see how much benefit you actually get from changing from air to nitrogen. The creators of this are selling it as a safety issue. I'd hate to be scammed into paying for premium air. I'm seeing Starbucks-style gasoline boutiques in our near future. What do you guys think? -- Rob

TOM: My first thought is, I'm putting all my money into Airbucks!

RAY: Like many sales pitches, the nitrogen idea has a molecule of truth in it. You're right that normal atmospheric air is about 80 percent nitrogen already. The rest is made up of oxygen, argon, water vapor, cat dander, bad breath and coal-plant particulates. And the truth is, atmospheric air is absolutely good enough for filling your tires.

TOM: Pure nitrogen has a couple of advantages. One is that it expands and contracts less under hot and cold temperatures than a mixture of nitrogen, oxygen and water vapor. That can be an advantage if you're in, say, a race car driving at 200 mph around a track for 500 miles -- where tiny differences in tire pressure and handling can really matter.

RAY: Nitrogen also doesn't support combustion like oxygen does. So it's unlikely to fuel a fire started in some other part of the car if a tire explodes. Of course, there's already plenty of combustible air all around the car, regardless of what's in the tires.

TOM: And finally, both the oxygen and the small percentage of moisture in the atmospheric air can contribute to degradation of the inside of your tires and wheels. But think about it: The outsides are exposed to the air all the time, so what are you worried about the insides for?

RAY: So, none of these advantages is important to the average driver. They just don't matter enough to ever think about. And they certainly don't matter enough to pay for, Rob.