A member of the Dodge Challenger owners' forum was buying a new car from a dealer and noticed green valve-stem caps on all four tires. The salesman told him that the tires had been filled with nitrogen, which would keep the tire pressure and temperature more consistent and that it would prevent tire rot from the inside out. It wasn't a free add-on, though. The "nitrogen upgrade" was a $69 item on the supplemental window sticker. Another forum member later posted that his dealer was charging $179 for this same "upgrade."
Some dealerships and tire stores claim that filling your tires with nitrogen will save you money on gas while offering better performance than air. But a closer look reveals that nitrogen has few benefits and much higher costs. For starters, a typical nitrogen fill-up will cost you about $6 per tire.
The Get Nitrogen Institute Web site says that with nitrogen tire inflation, drivers will note improvements in a vehicle's handling, fuel efficiency and tire life. All this is achieved through better tire-pressure retention, improved fuel economy and cooler-running tire temperatures, the institute says.
This sounds great in theory but let's take a closer look at each of those claims.
- Better tire-pressure retention: Over time, a tire will gradually lose pressure. Changes in temperature will accelerate this. The general rule of thumb is a loss of 1 psi for every 10-degree rise or fall in temperature. The institute says that nitrogen has a more stable pressure, since it has larger molecules than oxygen that are less likely to seep through the permeable tire walls.
In 2006, Consumer Reports conducted a year-long study to determine how much air loss was experienced in tires filled with nitrogen versus those filled with air. The results showed that nitrogen did reduce pressure loss over time, but it was only a 1.3 psi difference from air-filled tires. Among 31 pairs of tires, the average loss of air-filled tires was 3.5 psi from the initial 30 psi setting. Nitrogen-filled tires lost an average of 2.2 psi from the initial setting. Nitrogen won the test, but not by a significant margin.
- Improved fuel economy: The EPA says that under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires. The theory is that since nitrogen loses pressure at a slower rate than air, you are more likely to be at the correct psi and therefore get better fuel economy.
If you are proactive and check your tire pressure at least once a month, you can offset this difference with free air, and you won't need expensive nitrogen. We think this invalidates the "better fuel economy with nitrogen" argument.
For many people, however, this kind of maintenance is easier said than done. Most people either forget to regularly check and top off their tires, or never learned how to do it in the first place. Even Edmunds employees (typically a pretty car-savvy group) were under-inflating or over-inflating their tires, according to a tire-pressure study we conducted a few years ago.
And though tire-pressure monitoring systems (TPMS) now come standard on cars, a 2009 National Highway Transportation Safety Administration (NHTSA) study found that only 57 percent of vehicles with TPMS had the correct tire pressure. That's because most systems are only meant to signal that a tire has very low pressure, not to show that the pressure is optimal.
- Cooler running temperatures: When air is pressurized, the humidity in it condenses to a liquid and collects in the air storage tank you use at the local gas station. When you add compressed air to the tire, the water comes along for the ride. As the tire heats up during driving, that water changes to a gas, which then expands, increasing tire pressure. Because nitrogen is dry, there is no water in the tire to contribute to pressure fluctuations.
But this fluctuation in temperature isn't as significant as you might think. A 2008 ExxonMobil study plotted the changes in temperature over the course of various inflation pressures. The lines on the graph were virtually on top of each other. In other words, the change in temperature when using nitrogen was negligible.
- Prevent wheel rot: Nitrogen proponents will also point out that water in a tire can lead to wheel rot. A tire engineer who anonymously maintains Barry's Tire Tech, a blog on a number of tire issues, says this isn't really a problem with modern cars.
"Alloy wheels don't really have a problem with water inside the tire," the engineer writes in a post on nitrogen inflation. "They are coated to keep aluminum from forming aluminum oxide, which forms a crust, which isn't very attractive. But even then, this crust protects the aluminum from further corrosion from the water."
Where wheels have problems is when the aluminum alloy contacts steel, such as the steel spring clip used on wheel weights. It's a particular issue when salt is present, the engineer writes. "But this problem is totally independent of the inflation gas," he says. "Steel wheels only have a problem if the paint is damaged."