Unlike coolant, petrol doesn’t really last all that well. That’s because it’s a refined product from crude oil, and had to undergo fractional distillation to exist; it doesn’t occur naturally in nature. And, because nature is nature, petrol will degrade over time – generally anywhere from three to six months.
This is due to one of the very things we need from petrol – its volatility. Volatility means a lot of things in various contexts, and has been conflated in a lot of cases with ‘flammable’. But what it really means, at least in this context, is that it evaporates easily. Logic would suggest that this is an odd thing to desire in an expensive liquid, but think about how the fuel/air mixture in an engine works – aerating petrol in air. If the petrol stays together in drops, it’ll burn less effectively than if it turns to vapour. In fact, fuel injectors work to disperse fuel in infinitesimal droplets, to increase the surface area of the petrol and allow its natural volatility to work more quickly and evenly.
But, if petrol is left in your tank for months, these volatile compounds in the petrol will… well, be volatile, and evaporate. What’s left in the tank reacts with the oxygen in the air around it and oxidises, forming gums and varnishes and things that are much worse in an engine than petrol. These will clog filters, injectors, and fuel lines, and generally ensure that you have a bad time. So, the petrol in your tank will go from trouble-free fuel to an almost-flammable, contaminated mess in the space of six months. And with this in mind, we’re going to recommend that… you fill up your tank as soon as possible. It may sound mad, but fuel pumps, gaskets and lines rely on having petrol in them. Left to go dry, they’ll seize, rot and perish. Plus, with more air in the tank, there’s more surface area for oxidisation and space for condensation to form. And you don’t want either of those things. The best bet, once you have a full fuel tank, is to pour in some fuel stabiliser, which protects the fuel against evaporation and oxidisation for up to two years, depending on the manufacturer.
Diesel lasts a lot better, due to its comparatively low level of refinement. It’ll last for up to a year with no intervention, but it’s still best to fill up, because condensation can form inside half-empty fuel tanks. This applies to petrol as well, but there are much worse problems from oxidation. Keep your diesel car’s tank brimmed, and consider a diesel fuel stabiliser, which keeps water and oxidation to a minimum. Diesel will oxidise, albeit more slowly than petrol. But do you really want to tangle with a common-rail diesel system when you could avoid it by pouring a little stabiliser into a tank of fresh fuel and leaving it at that?
In both cases, pour in the additive as soon as you fill up, and ideally drive a decent distance – enough to warm the car up – before parking in the garage. This ensures that good, stable fuel is in your lines, pump and filter.
If you drive an electric car, take this opportunity to fully charge your car and leave it plugged in. Onboard systems can manage the charge level and ensure the battery remains in top condition – you won’t constantly be drawing charge. Smarter chargers can be set up to draw a tiny bit of charge late at night when the tariffs and usage are low, or set to draw down solar power from your home’s set-up on a sunny day.