Is there a cheapest day of the week to buy fuel? We checked. Sort of not.

You've probably heard it. Fill up on a Tuesday. Never buy fuel on a Friday. Supermarkets drop their prices on a Wednesday evening. Everyone has a theory, usually passed on by a mate who heard it from someone who used to work at a garage.

We're in a slightly unusual position: we can actually check. Every time a UK forecourt reports a new price to the Government's Fuel Finder scheme, we log it. Since February that's added up to 334,014 price changes for petrol alone, across 36 weeks and every forecourt in the country. So we sat down with all of it and asked a simple question. Does the day you fill up make any difference?

The short answer

Barely.

Sunday is the cheapest day to buy petrol. Friday is the dearest. And the difference between them is 0.15p a litre.

Let's put that in pocket terms. Fill a 50-litre tank on the cheapest day instead of the dearest, every single week, and you'll be about 7p better off. Per tank. Not £7. Seven pence. Over a year of weekly fills that's roughly the price of a packet of crisps.

Diesel is the same story with a slightly bigger gap of 0.19p, which gets you to 9p a tank. Still not worth setting an alarm for.

How we worked it out, and why it's trickier than it looks

Here's the thing that catches most "cheapest day" claims out. Fuel prices drift. Since February the national average for petrol has gone from about 136p to about 175p, with a dip in the middle. When prices are climbing, Monday will always look cheaper than Sunday for the boring reason that it came first. That isn't a day-of-the-week effect. It's just the trend.

So for every single day we compared the national average with the average of the seven days around it. That strips the trend out and leaves only the bit that's about the day itself. When you do that, this is what's left for petrol:

Those numbers are real, they're consistent across both petrol and diesel, and they're tiny. If someone tells you a particular weekday saves them real money, they're either remembering one good week or they've changed where they buy without noticing.

The bit that actually surprised us

Forecourts don't work to a weekly rhythm at all. They work to a weekday one.

Here's how many petrol price changes we logged on each day of the week:

Friday is the busiest day for repricing. Then the weekend goes quiet, with Sunday seeing less than a third of a normal weekday's changes. In other words, if you fill up on a Sunday, you're almost certainly paying Friday's price. Nothing much happens until the office opens again on Monday morning.

That's why Sunday comes out marginally cheapest. It isn't that anyone cuts prices for the weekend. It's that in a year when prices mostly went up, the weekend is when they paused.

One more number from the same table, because it tells you a lot about this year: of all those price changes, around three quarters were rises. On Sundays it was 84%.

So what does save you money?

If the day of the week is worth pence, where you fill up is worth pounds. Same data, much bigger numbers.

Supermarkets. Right now supermarket forecourts are 2.7p a litre cheaper than other forecourts on average. On a 50-litre tank that's £1.35, every time, and you don't have to do anything clever to get it.

The forecourt you drive past. On most of our town pages, the gap between the cheapest and the dearest station within ten miles is somewhere between 8p and 10p a litre. That's £4 or £5 a tank for going a couple of minutes out of your way. It is, by a mile, the biggest saving available to most people.

Motorway services. Motorway forecourts charge about 18p a litre more than everyone else. That's £9 on a tank. If you can make it to the next junction, do.

Which way prices are heading. This is the one place timing genuinely matters. In a rising week prices move by a couple of tenths of a penny a day, so filling up on Monday rather than Friday does save you something. In a falling week it's the other way round and waiting pays. The trick is knowing which kind of week you're in, which is exactly what our weekly report is for.

The honest conclusion

There is no cheapest day of the week to buy fuel. Not one that matters, anyway. Sunday wins on a technicality, by less than the rounding on the pump.

Fill up when it's convenient. Then spend the thirty seconds you'd have spent planning your week on checking which forecourt near you is cheapest today. That's where the money is.

How we did this: we rebuilt the national average price for every day since February 2026 from every logged price change, then compared each day with the average of the seven days centred on it. Price changes of more than 30p in one go were excluded as they are almost always typos in the feed. You're welcome to quote any of these figures; a link back is all we ask.