The best time to buy fuel
There is a lot of advice about when to fill up. Some of it is sound, and some has been repeated so often it is taken as fact despite being wrong by a factor of about a hundred.
The cold-morning myth
The most durable piece of fuel folklore is that filling up early, when the fuel is cold and therefore denser, gets you more energy for your money.
The physics is real: petrol does expand as it warms, by roughly 0.1% per degree Celsius. The problem is where the fuel is. It sits in tanks buried underground, where the temperature barely moves between dawn and afternoon — a degree or so at most.
A one-degree difference on a 55-litre fill is about 0.05 litres, or under 10p. That is smaller than the difference between two forecourts on the same road, and far smaller than the fuel used driving somewhere at a specific hour to get it.
Day of the week
This one has more to it, though less than the advice suggests. Retailers do tend to reprice at the start of the working week, and price rises have historically been more common heading into a weekend or a bank holiday when demand is higher.
But the effect is small and inconsistent between chains, and it is swamped by which forecourt you choose. Picking a cheaper site on a Friday beats picking a dear one on a Tuesday, every time.
What genuinely moves prices
Pump prices follow wholesale costs, with a lag. When oil falls, prices at the pump tend to come down slowly; when oil rises they tend to go up quickly. That asymmetry — often called rockets and feathers — is well documented and is one of the reasons the CMA pushed for open pricing data.
The practical consequence is that after a sustained fall in oil prices there is usually a window where some retailers have passed it on and others have not. That is a real opportunity, and it is visible: it shows up as the spread between forecourts widening.
The advice that actually works
Do not plan your week around fuel. Instead, know what the forecourts on your normal routes charge, and fill up at the cheap ones when you are passing anyway.
Fill up before a motorway journey rather than during it. And avoid running the tank so low that you have to take whatever is nearest — that is the single most expensive habit in fuel buying, because it removes the choice entirely.
Common questions
Is it better to buy petrol in the morning?
No, not meaningfully. Fuel is stored in underground tanks whose temperature barely changes through the day, so the density difference is worth pennies at most — far less than choosing a cheaper forecourt.
What day is petrol cheapest in the UK?
There is a mild tendency for repricing early in the working week and for rises before weekends and bank holidays, but the effect is small and varies by retailer. Which forecourt you use matters far more than which day.
Should I fill up before a price rise is announced?
UK pump prices are not announced in advance, so there is nothing reliable to act on. What is visible is the current price at every forecourt, which is a better basis for a decision.