Myth-Busting: Should you fill your car tyres with nitrogen?
Plenty of tyre retailers offer to fill your car tyres with nitrogen for a few bucks claiming it’ll improve fuel efficiency. So should you fill your car tyres with nitrogen?
FILLING YOUR CAR TYRES with nitrogen is a bit of a buzz thing to do. Race cars run nitrogen filled tyres and so do aeroplanes because it’s more stable in those high-pressure situation and, so, the argument goes, everyone should be filling their car tyres with it. And, as we’re discovering with our myth-busting series there’s a grain of truth to the argument.
Let’s start at the beginning. Air is made of 78% nitrogen, 21% oxygen and 1% other gasses. Surf the internet and you’ll find loads of article extolling the virtues of filling car tyres with ‘pure’ nitrogen, and that is any mixture with 90% nitrogen and less than 10% oxygen. From claims to improved fuel efficiency via both rolling resistance and inflation pressure loss.
And it’s true, filling tyres with nitrogen will see them hold their pressure for longer than an air-filled tyre and that’s because the oxygen content oxidises faster than nitrogen, leaking out of the rubber tyre, resulting in pressure loss. But this pressure loss only becomes an issue if you fail to properly maintain your tyres, meaning if you’re one of those people that don’t check your tyres from one end of the year to the next then, well, frankly, you’re a bit of a dill. See, an underinflated tyre is both dangerous and causing you to use more fuel, it’s also reducing the life of your tyre. So, for people like that, yes, filling with nitrogen because of its slower rate of oxidisation (because of less air in the mix) makes sense…or they could just take 5min to check their tyres once a week or even once a fortnight.
There aren’t many scientific studies into the benefits of running nitrogen in car tyres which is odd given the amount of businesses offering to fill you tyres with nitrogen, but they do exist. Indeed, we found one by the US National Highway Traffic Safety Administration, it’s like ANCAP and all the state and territory road authorities rolled into one. The study was conducted in 2009 and saw researches run a 90-day static lab test, as well as oven testing to simulate road running and three years of life. Tyres studied were a mix of general car tyres and light truck tyres.
The study found that after 90-days the 99% pure nitrogen-filled tyres lost around 1.39% of pressure/month while the air-filled tyres lost double that at 2.13%/month. However, the study found that if, after the 90 days the air-filled tyre was again filled with air its pressure drop over another 90 days would fall to 1.59%. This is because while the oxygen content in the air mixture was oxidising in the tyre, the nitrogen content remained static but increased when the tyre was topped up. “the approximate predicted inflation pressure loss rates for the air-inflated tires, now containing 85 percent N2, would be 1.57 percent/month (0.94 psi/month),” the study found.
In conclusion, over a three-year period, the NHTSA stated the advantage of nitrogen filled tyres compared to air-filled tyres was around 17%, and that’s with 90-day intervals between tyres being topped up. The study also found that using nitrogen instead of air had no effect at all on the rolling resistance of the tyre with data, after 90 days of no inflation pressure maintenance resulted in air-filled tyres experiencing a 1.5% reduction in rolling resistance.
And, while it’s a known fact that oxygen has a degrading effect on rubber, the NHTSA concluded that after its 90-day static test and then accelerated oven ageing showed no difference between air-filled or nitrogen-filled tyres. That said, when an oxygen-rich (50:50) mix was tested then, yes, multiple failures were recorded after the static and oven-ageing test.
In the end, the NHTSA concluded, “It should be noted that inflation with nitrogen merely slows the rate of diffusion of gas from the tire and is not a substitute for regularly maintaining tire pressure”.
So, choosing to fill your tyres with nitrogen comes down to what kind of car owner you are. If you’re lazy and not interested in keeping an eye on your tyres (which you should be doing) then go ahead and spend the money to fill your car and 4×4 tyres with nitrogen. But for those who keep an eye on tyre pressures and top up their tyres when they drop, then running nitrogen in your car tyres is a waste of time. More than that, the argument that nitrogen-filled tyres will get better fuel efficiency because of less pressure drop over time is largely irrelevant as an argument because the pressure loss it minimal.