Nitrogen in Tyres (Tires) vs Air — What NHTSA Tests Found

Nitrogen vs air in tyres at a glance

Nitrogen in a tyre makes one measurable difference: it leaks out more slowly than air. In NHTSA lab tests, tyres filled with nitrogen lost pressure at roughly two-thirds the rate of tyres filled with air, and the gas itself had no direct effect on rolling resistance. Ordinary air is already mostly nitrogen. Having the right pressure, checked regularly, matters much more than which gas is inside.

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The only official test data on this comes from the US: NHTSA report DOT HS 811 094 and a US Government Accountability Office (GAO) review. Tyres behave the same way in the UK, Canada and Australia, so the findings below apply to drivers there too.

Why nitrogen leaks more slowly than air

Oxygen passes through tyre rubber faster than nitrogen does. If you take the oxygen out of the fill, less gas escapes through the tyre wall. NHTSA found that this advantage did not depend on the tyre type, the inflation pressure, or the thickness and make-up of the inner liner.

Air-filled tyres also drift towards nitrogen over time without any special fill. NHTSA measured tyres from cars that were filled and topped up with air. After about three years of use, the oxygen in them had dropped and the nitrogen share had risen. This is why lab tests on new tyres overstate the benefit for tyres in daily use.

Does nitrogen improve fuel economy?

Not directly. NHTSA found that filling with nitrogen instead of air had no immediate effect on rolling resistance in the lab. The only fuel benefit is indirect: a tyre that holds its pressure longer rolls more easily.

Pressure itself matters much more. In NHTSA’s data, rolling resistance on car tyres rose with every psi lost. The GAO notes that tyre experts expect air-filled tyres to lose some pressure every month through the rubber alone. If your gauge reads in bar or kPa, the tyre pressure converter translates the figures. A monthly top-up with air recovers more than nitrogen’s slower leak ever saves.

Does nitrogen make tyres last longer?

Oxygen ages tyres from the inside. NHTSA examined tyres taken off cars after years in Phoenix, Arizona, and found that oxygen seeping through the tyre had reacted with the rubber between the steel belts and weakened the bond between them. In an oven test with extra oxygen in the tyre, a number of tyres failed the endurance test afterwards.

In the same oven test, every tyre filled with ordinary air or with nitrogen passed. NHTSA says the test may have been too short to separate air from nitrogen. It also says there is a “strong scientific basis” that less oxygen slows ageing inside the tyre. That is a reasonable expectation, not proven extra tyre life.

Ageing from oxygen matters most for tyres that sit for months, such as off-season sets and spares. For those, storage conditions matter at least as much. See tyre storage tips for the seasonal change.

Topping up a nitrogen-filled tyre with air

You can safely top up a nitrogen-filled tyre with air. Air is mostly nitrogen, so adding it only dilutes the fill. NHTSA’s figures show the benefit simply shrinks as the mix gets closer to normal air. A tyre at the right pressure with mixed gas is much better than an underinflated tyre kept “pure”.

Warning lights don’t remove the need to check. In the US, new cars, SUVs and pickups from the 2008 model year must have a tyre pressure monitoring system (TPMS). It warns only when a tyre is well below the recommended pressure, which is past the point where grip and economy suffer. This applies whether the tyre holds air or nitrogen.

What to do: pay for nitrogen or stick with air?

• Nitrogen included free with new tyres: take it, but keep checking pressures on your normal schedule.
• Charged per tyre: what you pay for is a slower leak. Regular checks with air get the same result.
• Car that rarely moves, stored tyres or a spare: this is where a slower leak and less oxygen are most useful, because the tyres go long periods without checks.
• Away from home with no nitrogen nearby: fill with air to the pressure on the door placard or in the handbook. It does no harm.
• Electric car: the same rules apply. Heavier EVs make correct pressure more important, not the gas. See the EV tyre guide.
• Either way: check pressure with the tyres cold and set it to the vehicle maker’s figure. The tyre pressure guide explains how.

Sources

• National Highway Traffic Safety Administration, The Effects of Varying the Levels of Nitrogen in the Inflation Gas of Tires on Laboratory Test Performance, report DOT HS 811 094 (2009).
• US Government Accountability Office, Underinflated Tires in the United States, GAO-07-246R (2007).