How Long Does Fuel Last In Your Car Before It Goes Bad?

Got your wires properly crossed there, Jessica Choksey, from JD Power. Well done, advising people to keep their tanks topped off. Don’t do that, and this is why.

Fuel does react with atmospheric oxygen - it oxidises, sure - but really, really slowly. There is very little oxygen in regular, normal air we breathe, only about 21 per cent. One litre in every five, basically.

And if you’ve got 25 litres of fuel in a 50-litre tank, you’ve actually got about 30 litres of air (because there’s also an air space for expansion). And 25 litres of fuel weighs about 19 kilos, which requires about 280 kilos of air to oxidise fully (at 14.7 to one).

Because air weighs about 1.2 kilos per 1000 litres, you would need, roughly, a quarter of a million litres of air to oxidise the 25 litres of fuel completely. And you’ve only got 30 litres of air in the tank. So that’s enough to oxidise roughly bugger all of your fuel; about 120 parts per million. So, don’t waste your time worrying about this.

Even if there’s water in the tank from condensation or other effects adding to oxidation, it’s a minor effect in the context of a car that’s not getting used as much as it was, pre-COVID.

As for evaporation, this is when fuel vapour rises up and saturates the air (not the oxygen, Jessica - back to school for you). But once that air gets saturated with fuel vapour in the tank, evaporation stops. That’s how this works. The air in the tank is a sponge, and a sponge can only hold so much water. The fuel can’t just run away evaporating, because the fuel system is sealed. By law.

It can keep evaporating in your busted-arse lawnmower, most probably. But not your car.

So, look at it like this, dude: Say one day we have a big pandemic, all around the world, and you go from commuting every day and filling up twice a week, to just driving to the shops twice a week so you don’t get cabin fever, and to stock up on masks, hand sanitizer and toilet tissue. All of life’s essentials.

Say it’s a five-kay round trip. That’s 10 kays a week of driving. 520 kays every year. About one tank of fuel in an average car.

Every 12 weeks you’ll get down to a quarter of a tank. So stop at the servo, on the way home, every three months, buy a quarter of a tank - live life on the edge. Mask up. Fill up. Keep JD Power Jessica happy - and her quaint oxygen evaporation hypothesis. This way, your tank will be a shandy of new petrol, three-month-old petrol, six-month-old petrol and nine-month-old petrol. Kinda thing.

It’s highly unlikely you will experience any operational problems whatsoever. Think about boobies instead.