Dear Tom and Ray:

I heard a story on my local NPR station about a place in Houston where car
owners can have the air in their tires replaced with nitrogen at a cost of
$100. This service is advertised as a fuel economy enhancement because nitrogen
diffuses through the tire at a slower rate than air, and so the tires stay at
the proper pressure longer. Is this a scam, or what? -- Mark

RAY: Yes. And a very clever one at that. In fact, my brother's on the phone
right now seeing if we can get the local nitrogen-transfusion franchise around
here.

TOM: Their claim that nitrogen keeps your tire pressure more constant is
actually correct. But their reasoning is wacko. It has nothing to do with
diffusion of air through the rubber.

RAY: The reason nitrogen pressure stays more constant than air pressure is
because air contains water vapor, and so it expands less predictably than
nitrogen. There's a different percentage of water vapor in the air on any given
day (a k a the humidity), so you never know exactly how much tire expansion
you'll get from the water vapor.

TOM: It's not enough expansion for you or me to notice, but some race-car
drivers notice it. Formula One (i.e., Indy 500) racers use nitrogen in their
tires because when you're traveling around an oval track at 200 mph, you want
your tire pressure to be entirely predictable. They even "stagger" the tire
pressure on those cars, making the outside tires a little fuller than the
inside tires to keep the car turning inward. And at those speeds, an eighth of
an inch in tire height can make a huge difference.

RAY: But for you and me (and everyone else reading this column today), Mark, it
makes absolutely no difference at all. And it's simply not worth it (unless, of
course, you can steal the nitrogen from work!).

TOM: Seriously, you can keep your tire pressure constant enough for street and
highway driving by simply checking it periodically.

* * *

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