Kia Soul green cap shows there is nitrogen in the car tires

This common pitch is compelling: Filling your car’s tires with nitrogen will reduce air loss, boost fuel economy, reduce rolling resistance, and improve safety. Some cars are even sold with the telltale green caps on valve stems, signaling the tires have already been filled with nitrogen.

Dealerships and tire shops often charge owners to fill tires with nitrogen, at $5 or more per tire, during routine service visits or when replacing tires. But is nitrogen worth the price?

Testing conducted independently by Consumer Reports and the National Highway Traffic Safety Administration show the benefits are more theoretical than practical.

Tire Ratings

First off, dry air is 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases—and there is almost always some water vapor present. The nitrogen used to fill tires is a processed gas that's void of many of the impurities in regular air, but if you plan on using it, make sure the retailer removes all the original air to ensure the tire is truly filled with nitrogen. Also, realize now that you’ll want to top the tires off with nitrogen in the future. (There is no harm in mixing in regular air, but it does defeat the purpose.)

“Most of the selling points for nitrogen really relate to maintaining proper tire pressure,” explains Gene Petersen, who runs CR’s tire testing program.

But do tires retain their inflation better with nitrogen than with regular air?  


See our tire retailer ratings.
 

How We Tested Air Loss

Consumer Reports conducted a 12-month study at our Auto Test Center using 31 tire models of all-season tires. The tires and wheels were purchased new, and they were used for our 16,000-mile tread wear test.

We first ensured that we had 95 percent nitrogen purity in the tire—the claimed limit of our nitrogen system, which generates nitrogen gas from ambient air and removes water vapor.

We filled one tire per model with air and another with nitrogen. Both were filled to 30 psi (pounds per square inch) at room temperature. We set both tires outdoors for one year, and then checked the inflation pressure at room temperature again.  

More on Tires

Both tires lost pressure over that time, but the difference in loss was minimal. The average loss of air-filled tires was just 3.5 psi from the initial 30 psi setting. Nitrogen-filled tires lost an average of 2.2 psi.

The results show nitrogen does reduce pressure loss, but the reduction was only a 1.3 psi difference from air-filled tires over an entire year.

“Our test showed that while nitrogen has some advantages, both nitrogen and air-filled tires lose pressure over time,” Petersen says, “It is important that car owners routinely check their tire pressure.”

We have concerns that car owners who use nitrogen might check their tire pressures less often. That would be a mistake, Petersen says.  


See our tire buying guide.
 

What NHTSA Found

The NHTSA study also found that regular air escapes tires at a higher rate than pure nitrogen.

The government agency concluded that using nitrogen does reduce tire degradation by limiting oxidation. Oxidation refers to the damage caused by exposing tires to the oxygen in regular air, which can make rubber become brittle. However, this is not a practical concern for most consumers, because they replace the tires long before degradation is a problem. 

Tire Tools

In our experience, passenger tire service life is dependent on both wear (how even it is and how many miles a tire goes) and age. Tires are designed to be used with air, and degradation is not a problem for regular tires used and maintained properly. Some tire manufacturers say tires need to be replaced within 10 years—even if they’ve never been used—because of how the rubber degrades over time.

The lab report from NHTSA also found that “inflating with nitrogen in place of air had little or no direct effect on tire rolling resistance performance.” Rolling resistance is a factor in fuel economy, and it is something that is part of CR’s routine tire testing. The underlying concern is that an underinflated tire would have increased rolling resistance, hurting a vehicle’s fuel economy. Nitrogen may aid this because it seeps out at a slower rate. But with either gas, the tire’s rolling resistance would be the same when maintained at the proper tire pressure.

Bottom Line

As anyone who has had a flat tire can explain, tires don’t hold up a car by themselves: They need to be filled with air. Pressure changes over time, with temperatures affecting the air density and the air permeating the rubber. Check tire pressure once a month, when the tires are cool in the morning, and top off as necessary with regular air. It’s a low-cost way to bolster your car’s safety and fuel efficiency.

“Routine maintenance is critical to long tire life and good health. Proper tire inflation pressure is the objective,” says Woody Rogers, director of tire information at Tire Rack, a major national tire retailer. “Air works fine and is essentially free.”

There is a stronger case to be made for nitrogen for high-tire-pressure, heavy-duty applications, ranging from large commercial trucks to airplanes. But based on our experience with passenger cars and these test results, consumers would be better served—and would save their money—by just using air in their tires and checking them monthly.

There is nothing wrong with using nitrogen in passenger cars and trucks, but it does take commitment and potentially cost. Nitrogen users should be diligent about only topping off with nitrogen to ensure they are getting the intended benefits.