Is there really a cheapest day to fill up?

Fill up on Monday. Never on a Friday. Go early, before the station raises prices for the day. You have probably heard at least one of these, repeated with total confidence. The problem is that they treat a national, slow-moving market as if it changed on a neat weekly timer. It rarely does, and when it does, the effect is far smaller than the difference between two stations you could reach right now.

Worse, chasing a rule of thumb can send you to a pump that is habitually more expensive, on the very day you were told it would be cheap. To beat the average you have to know what actually sets the number on the sign.

The price you pay is mostly not the fuel. A large, fixed slice is tax: excise duty plus VAT, set by your country and unchanged from Monday to Sunday. On top of that sits the wholesale cost, which tracks crude oil and refining margins on international markets, and finally a thin retailer margin that pays for the site, the staff and the card fees.

Because the wholesale part moves on global markets and is passed through with a lag of days, not hours, a station's price tends to drift rather than jump on a schedule. When oil rises, pumps climb over the following days; when it falls, they ease down, often more slowly. None of that follows your calendar.

This is why two identical cars, filled in the same city on the same afternoon, can pay noticeably different prices: same tax, same wholesale cost, different retailer and different local competition.

Sometimes, in some markets, and always weakly. In a few countries retailers do nudge prices up before busy weekends or holidays, when demand is highest and drivers are least price-sensitive. A handful of markets are regulated in ways that create daily or weekly rhythms. But these patterns are local, they change, and they are usually worth a cent or two, not the 15 to 20 cents a litre you routinely find between neighbouring stations.

So a weekly rule can be right on average and still lose you money on the day, because it ignores the far bigger, always-present gap: which station, not which day.

Line up the stations within a few kilometres of you and the spread is striking. It is common to see the cheapest and the most expensive differ by 15 to 20 cents a litre at the very same moment. On a 50-litre fill that is 7 to 10 euros, every single time, and it has nothing to do with the day of the week.

That is the money worth chasing, and it is the one thing folklore can never tell you, because it changes street by street and updates constantly. The only way to capture it is to look at the real, current prices around you.

Forget the calendar and work the levers that actually move your yearly fuel bill:

• Compare before you pull in. Check the real prices of the stations you can reach right now, rather than trusting a habit or a single familiar pump.
• Count the detour. A station five minutes away with a lower price can cost you more once you burn fuel getting there. Fillz does this maths for you and shows the one that genuinely pays off.
• Never run near empty. When your tank hits the last quarter you lose all choice and fill wherever you happen to be. Keep a buffer so you are always picking, not settling.
• Fill before motorways and remote stretches. Prices climb where competition thins out. Top up in town before a long or rural drive.
• Use the discounts you already have. Brand or supermarket fuel cards can beat any day-of-week trick. Set your card once in Fillz and every result includes the discount in the total.

Is fuel really cheaper on Mondays?

Not reliably, and not everywhere. Any weekly pattern is small and varies by country, while the gap between nearby stations on the same day is usually far bigger. Comparing prices beats guessing the day.

Should I fill up early in the morning?

The old idea that cool morning fuel is denser makes no practical difference to what you pay. Stations do not reprice on a daily clock in most markets. Convenience matters more than the hour.

Do prices really change during the day?

Usually not on a fixed schedule. Retail prices drift with wholesale costs over days. A station can change its sign at any time, which is exactly why a live comparison beats a rule of thumb.

Is the cheapest station always the best choice?

No. Once you count the fuel spent driving to it, a slightly pricier station nearer you can win. That detour maths is the core of what Fillz calculates on every result.