Is nitrogen in your tyres a gimmick?
Some tyre fitters and dealers offer to fill your tyres with nitrogen instead of air, usually for a fee and with green valve caps to show for it. It isn't a scam, but the benefit on an ordinary car is a lot smaller than the pitch suggests.
Air is mostly nitrogen already
The air from any pump is about 78% nitrogen and 21% oxygen, with a little water vapour and other gases. A nitrogen fill pushes the nitrogen up to 93–95% or more. So you're not swapping one gas for another, you're taking most of the oxygen and moisture out.
What it's supposed to do
- Lose pressure more slowly. Oxygen seeps through tyre rubber a little faster than nitrogen does, so a tyre with less oxygen in it holds its pressure slightly longer.
- Keep the pressure steadier. Nitrogen fills are dry. Moisture in ordinary air makes the pressure swing a bit more as the tyre heats up and cools down.
- Less corrosion inside the wheel. Less oxygen and water means less rust or corrosion on the inside of the rim, in theory.
What the tests found
The best-known test, by the US magazine Consumer Reports, ran tyres for a year. Tyres filled with air lost an average of 3.5 psi; tyres filled with nitrogen lost 2.2 psi. So nitrogen did help, by about 1.3 psi over a whole year. If you check your tyres monthly, that difference never comes into it. If you never check them, nitrogen won't save you either.
Where nitrogen does make sense
Aircraft, racing cars and some heavy fleets use nitrogen because they need very consistent pressures across big temperature changes, and because a dry, non-flammable gas is safer in extreme conditions. Those are good reasons for them. For a family car doing the school run and the odd motorway trip, they don't really apply.
The catches
- It can cost money. Some places charge per tyre for the fill, and again for top-ups.
- Top-ups are harder to find. Most petrol station machines only do air.
- It doesn't stop slow punctures. A nail, a leaking valve or a bad seal on the rim will let nitrogen out just as fast as air.
Can you mix nitrogen and air?
Yes, completely safely. Topping up a nitrogen-filled tyre with ordinary air does no harm at all; it just dilutes whatever small benefit you paid for. Never drive on a low tyre because you can't find nitrogen.
The verdict
Nitrogen isn't harmful, and if it comes free with new tyres there's no reason to turn it down. But it isn't worth paying much for. The thing that actually keeps your tyres right is a gauge and two minutes once a month, with whatever gas is in them.