Nitrogen in Tires: Is It Worth It? A Myth Check

Colored caps and big claims

Some tires come back from a shop with nitrogen in them, sometimes marked with a colored valve cap. The pitch usually goes something like: steadier pressure, cooler running, longer tire life, better mileage.

Some of that has a real basis. Some of it gets stretched. Here's what tire makers and a vehicle maker actually say, so you can decide whether it's worth paying for.

What nitrogen is, and what's already in your tires

Ordinary air is already mostly nitrogen. Michelin describes nitrogen tire fill as dry air with the oxygen removed. So switching to nitrogen doesn't swap one gas for a completely different one. It raises the share of nitrogen and takes out most of the oxygen and the moisture.

That matters, because it keeps the debate in proportion. The difference between a nitrogen-filled tire and an air-filled tire is a difference of degree.

The real benefits, according to the tire makers

There are two genuine effects, and the tire makers acknowledge both.

It seeps out more slowly. Bridgestone explains that nitrogen molecules are larger than oxygen molecules, so nitrogen escapes more slowly through the tire's inner liner, and the tire holds its correct pressure longer.

It's dry. Bridgestone also notes that compressed air contains water vapor, which might promote corrosion and makes pressure vary more with temperature changes. Nitrogen fill is dry.

A General Motors service bulletin on the subject lists similar benefits under controlled conditions: less pressure loss over time, less pressure variation with temperature thanks to less water vapor, and less long-term rubber degradation from oxygen.

The real limits

Here's the part the pitch tends to skip.

Tire makers don't require it. Bridgestone calls nitrogen an acceptable inflation gas for its passenger, light truck and truck tires, says it has no comment on the various performance claims made by nitrogen suppliers, and notes that its own company-owned service centers use air as standard practice. Michelin says its tires are designed to deliver their expected performance with air or nitrogen, as long as the vehicle maker's recommended pressures are respected.

Top-ups undo much of it. The GM bulletin says the theoretical benefits are reduced in practical use, because there's no infrastructure to keep tires filled with nearly pure nitrogen, and that even occasional inflation with ordinary air negates many of the theoretical benefits. Its conclusion: the realized benefit to GM customers is expected to be minimal.

Tires leak in other places. Michelin points out that tires can lose pressure at the tire and rim interface, the valve, the valve and rim interface and the wheel itself, which no inflation gas can prevent. A slow leak from a nail is a slow leak with either.

It still changes with temperature. Nitrogen-filled tires still read higher when warm and lower when cool. Try the numbers here:

Can you mix nitrogen and air?

Yes. Michelin says nitrogen and compressed air mix without issue when you're adding pressure.

The practical rule: never drive on a low tire because the nitrogen machine is far away. Top up with air at the nearest pump. You'll dilute the nitrogen and give back some of its benefit, but correct pressure today is worth more than the right gas next week.

What actually moves the needle: pressure

Everyone in this debate agrees on one thing. Bridgestone says the most important part of tire maintenance is keeping the correct pressure, because properly inflated tires wear more evenly, stop and handle better, improve fuel economy and last longer.

The fuel-economy numbers come from the federal government's fueleconomy.gov:

• keeping tires at the proper pressure can improve gas mileage by 0.6% on average, and up to 3% in some cases;
• under-inflated tires can lower gas mileage by about 0.2% for every 1 psi drop in the average pressure of all tires.

Those gains come from correct pressure, not from nitrogen. A nitrogen-filled tire that's 5 psi low costs you the same as an air-filled tire that's 5 psi low.

So, is it worth it?

A balanced way to think about it:

We don't think less of shops that offer nitrogen, and some drivers like it. It just isn't a substitute for the gauge.

The routine that works with either gas

1. Check monthly and before long trips. That's Michelin's and NHTSA's guidance.
2. Check cold. USTMA defines cold as parked for at least three hours.
3. Set to the door placard, not the number on the sidewall. Our myth check on filling to the sidewall's max PSI explains why.
4. Don't let air out of warm tires. NHTSA's summer guidance makes this point: pressure rises as tires heat up, and drops back as they cool.
5. Investigate a tire that keeps losing air. That's a leak, not the gas. Our TPMS light guide covers what the dash warning means.

Our guide to tire pressure and temperature covers the seasonal swings. And if a tire won't hold pressure whatever you fill it with, have it inspected. Not sure your tires are due for more than air? Run them through Do I Need Tires Yet?. In South Florida, Extreme Tires checks and repairs leaks in Sunrise or by mobile van in the local install area.