Why some people fill their tires with nitrogen

One of the many special products offered to car owners to allow them to get more bang for their buck is nitrogen gas to fill their tires. But tires are tires. As long as they're not filled with gravel or anything that can explode, what difference could the gas inside them make? Supposedly, a lot. Let's examine the evidence.

One argument is that oxygen breaks seemingly everything it touches. It causes metal in the wheels of a car to rust. It makes the rubber in the tires go brittle and crack. It also, supposedly, catches fire more readily than nitrogen does. No one likes to be zooming along on four rapidly spinning fire hazards. It's also a lighter substance than oxygen, since nitrogen has one less proton than oxygen and hangs out in smaller molecules.

But the main advantage of nitrogen, according to believers, is tire pressure. Although nitrogen is lighter than oxygen, it has a tubbier structure. Oxygen and nitrogen both form diatomic molecules. They wander through the world in groups of two, each single atom joined together with another. These joinings don't result in big globs of material. Instead they are just two atoms connected at a certain point - like two people holding hands. The connection point - the clasped hands - is smaller in the diatomic oxygen molecule than it is in the diatomic nitrogen molecule. This means that more oxygen than nitrogen will trickle out through the rubber tire. What's more, nitrogen gas doesn't carry the same impurities as regular air. Air from a gas station may be up to five percent moisture, which can screw around with tire pressure. As a result, nitrogen is a better way to keep tires optimally inflated.

The truth about nitrogen

Or so they say. Nitrogen-filled tires are are only safer and more "green" if you assume that most people won't maintain the correct pressure in their tires. Having soft tires does waste gas, so it's tempting to seek out gasses that will keep them plump longer than oxygen. But you can get the same effect of nitrogen by checking the pressure on your tires regularly, and filling them with air every month. Even the slobs who re-fill with regular air once in a blue moon won't be doing that badly. The air in them is already 78% nitrogen.

Some even doubt the idea that nitrogen leaks appreciably less than regular air in the first place. Consumer Reports filled tires with regular air and with nitrogen gas, and compared the pressure after a year. The difference was a measly 1.3 pounds per square inch. The test was limited, since it consisted of tires being filled and then 'set outside' instead of driven for a year. Still, that's not an impressive showing for nitrogen. As for the corrosive effects of oxygen - most experts think that speeding down the highway or braking abruptly does more damage than a few floating molecules. It seems the main reason to fill tires with nitrogen is if you're a car person and you want bragging rights. But if you are a car person, you're probably checking your tire pressure regularly already.

Verdict: Nobody really needs nitrogen in their tires, except to sound fancy.

Via The Straight Dope, Consumer Reports, Get Nitrogen, The Naked Scientists, and Scientific American.