I was in Manhattan, Kan., waiting on my truck to be serviced about two years ago when the salesman at the service station brought me a brochure. It touted the benefits of filling my tires with nitrogen. I had never heard of such a practice, so I read the entire brochure with amazement.

It seems that there are companies that market nitrogen for use in automobile tires. The language on a brochure or a website from one of these companies reads just like that from any popular pseudoscience or scam. Phrases like “amazing benefits” and “high purity” could just as easily come from a vitamin supplement company. In the first couple of above-the-fold paragraphs on the website from one such popular company, it’s explained how the high cost of this technology – generating and delivering nitrogen – has kept it out of the hands and tires of the common folk until now. Lucky us. Seems  it was the college kids with the nitrogen tanks for their kegorators who’ve had the secret this whole time.

Like any good skeptic, I’m not going to base an entire case against a product on poorly selected marketing tactics. Let’s think about what the product is trying to solve, decide if it makes sense and, if so, then decide if it’s economically viable.

The benefits for using nitrogen in tires comprise increases in fuel economy, tire life and safety.

Fuel economy
The hypothesis is that because nitrogen gas molecules are larger than those of oxygen, the natural seepage is reduced. Because lower-than-proper tire pressure has a negative impact on fuel economy, nitrogen must be better.

Tire life
The hypothesis is that the double bonds in the rubber of tires are susceptible to oxidation and that, over time, the anti-oxidants used by tire manufacturers to prevent this oxidation are degraded to the point that the oxygen in the tires begins to attack the rubber. Also, the moisture in air can cause corrosion on the rims. Because nitrogen doesn’t oxidize the rubber or corrode the wheels, nitrogen must be better.

Safety 
The hypothesis is that because the expansion rate of nitrogen is less than that of air. Because varying pressures in tires can lead to problems in the consistency of handling, steering and braking, nitrogen must be better.

Let’s look at the difference between oxygen and nitrogen. I’m not a chemist, but from what I can find the difference in molecular size between the gasses of oxygen and nitrogen is about 3%. Furthermore, the air we breath – the same air that is compressed to fill most of our tires – contains nearly 80% nitrogen anyway.

Here’s some math from the Car Talk website.

Under-inflated tires lowers gas mileage by 0.4 percent for every one pound of drop in pressure of all four tires. So, if you’re down by 10 pounds… you’re losing 4 percent in fuel economy… For every drop of 10° Fahrenheit in air temperature, your tires will lose one pound of pressure.

I believe that nitrogen-filled tire pressure does change with temperature, perhaps just not as much as tires filled with air. If nitrogen pressure didn’t change at all with temperature, then by this math, properly inflating your tires with nitrogen during the summer can prevent this 4% loss in fuel economy if the temperature drops 100 degrees. Or you can simply check your tires every few months and adjust the pressure accordingly.

What about leakage? Does the 20% of the air in the tires that is 3% smaller than the rest really leak out so much faster that there is a change in pressure over the course of the few months between checks? I believe it may be slightly measurable – if using a highly accurate gauge – but I don’t believe it would be noticeable or significant to either the handling or the fuel economy.

We don’t drive our cars in a pure nitrogen environment, so the outside of our tires and our wheels are constantly exposed to oxygen, moisture and other more nefarious substances. I’ve never heard of anyone who had to replace a tire or rim due to holes rusting through from the inside of the wheels. Again, I suppose that it’s something that could be measured in the lab over the course of a very long time, but the wear on the tires will cause the need for a new tire long before the anti-oxidants (if this claim is even believable) go away. Likewise, the failure of other components on the car will cause the need for a new car long before the wheels corrode to the point that they need replaced.

The biggest logical fallacy noticeable in nearly all advertising for nitrogen in tires is an argument from authority. Because NASCAR and the US military use nitrogen in their tires, we average consumers should do the same. The high performance requirements – taking a turn at 150 mph or landing a bomber on a runway – require consistent, reliable air pressure. Both the military and NASCAR have large budgets, and at these extremes the slight benefit – only a percent or two based on the math above – of the nitrogen is worth the price which they don’t really care about anyway.

In summary, tires filled with air contain roughly 21% oxygen and 78% nitrogen and small amounts of water vapor and other trace gasses. Filling tires with dry nitrogen replaces that 21% oxygen with a molecule that is 3% larger and reduces moisture inside the tire. Does this practice increase fuel efficiency, tire and rim longevity, and safety? My guess is that it may be slightly measurable but certainly not noticeable – and most definitely not worth the additional cost for anyone who properly monitors tire pressure.

What did I miss? Let me know in the comments.