NASCAR and Indy Car teams use nitrogen because it allows them to more accurately predict tire pressure fluctuation

If you are one of the 85% of Americans who don't regularly check tire pressure, you need nitrogen.

We take in nitrogen with every breath. Air is composed of:

1% Water Vapor and Other Gases – Escapes up to 250 times faster than Nitrogen
21% Oxygen – Escapes 3-4 times faster than Nitrogen
78% Nitrogen – The largest molecule in air, dry, non-flammable.

Because of their large size, nitrogen molecules are the least permeable and stay in your tire longer.

Its not about the nitrogen.  It's about reducing oxygen, water vapor and other gases.

By reducing the percentage of oxygen, water vapor and other gases in your tires from 22% to 7% or lower, your tires will maintain proper pressure longer than if you use “plain old air.” For example, with 95% nitrogen in your tires, they retain optimal pressure three to four times longer.

Proper tire pressure is a big deal. Maintain it with nitrogen, and you'll see
these three primary benefits:

Increased Fuel Efficiency - Correct tire pressure keeps the manufacturer's recommended "contact patch" on the road.  This lessens the rolling resistance and maximizes fuel efficiency. 

Longer Tire Life - When it comes in contact with other materials, oxygen cuases oxidation.  Oxidation can make rubber brittle and cause it to lose tensile strength.  In addition, at high temperatures and pressures, oxygen reacts and damages inner tire liners and belt packages; nitrogen does not.

Increased Safety - Under-inflated tires cause 90% of blowouts.  Nitrogen provides more reliable pressure for reduced blowout potential. 

Other benefits:

Improved TPMS Performance - If you have a new car, you likely are plagued by a flashing light telling you your tire pressure is low.  for example, one woman's light was going off every four to five weeks.  After inflating with nitrogen, her light didn't reappear for 53 weeks.

More Predicatable Pressure Fluctuation - NASCAR teams use nitrogen so they can more accurately predict tire pressure fluctuation.  Regular compressed air can fluctuate considerably when water vapor is present.

Longer Rim Life - Rim rust caused by condensation from water vapor and other gases can get caught in valves and create slow leaks in tires.  Nitrogen is completely dry, so it eliminates the potential for condensation.

Why not eliminate all oxygen and water vapor?
What's right for me - 95% or 98%?

Numerous studies have proven that nitrogen in tires reduces the volume of gases that escape more quickly and cause damaging oxidation.  However, research also has shown that nitrogen purity beyond a certain point does not provide additional benefits.  In fact, with a nitrogen purity above 93.4% in passenger tires, oxygen actually begins to migrate back into the tire.  You can get all the benefits of nitrogen with a purity level between 93% - 98%.

Bridgestone/Firestone researchers say that 93 - 95% nitrogen is all you need.

According to Ford Motor Co., there is no difference between 96% and 99% nitrogen purity.

Who Else is using Nitrogen?

NASCAR-NASCAR teams use nitrogen because it allows themm to more accurately predict tire pressure fluctuation.  Nitrogen fluctuates with temperature change, but it does so less than when water vapor is present.  In addition, higher nitrogen levels eliminate the explosive properties of oxygen (Oxygen loses its explosive properties at around 9% or less) NASCAR uses bottled nitrogen for portability.  The bottles are dielivered to the track by Praxair.

Commercial Airlines - The Federal Aviation Administration (FAA) requires nitrogen in all commercial aircraft tires to eliminate the potential for water vapor (inherent in normal compressed air) from freezing at high altitudes.  In addition, aircraft manufacturers such as Boeing us nitrogen membranes in their On-Board Inert Gas Generation Systems (OBIGGS) to "top" fuel tanks with nitrogen -an inert gas that does not support combustion.

U.S. Government  - NASA and the U.S. military use nitrogen for many of the same reasons it is used in commercial aircraft.

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