Supermarket fuel versus branded fuel
The price gap between a supermarket forecourt and a branded one is large and consistent. The quality gap is far smaller than the price gap suggests.
The fuel is held to the same standard
All petrol sold in the UK must meet BS EN 228, and all diesel BS EN 590. These are not loose guidelines — they specify octane, sulphur content, volatility and much else, and they apply regardless of the sign above the forecourt.
Fuel is also frequently distributed from shared terminals. Two tankers leaving the same depot, one heading to a supermarket and one to a branded site, may be carrying the same base product.
What actually differs
The difference is the additive package blended in at the terminal. Branded fuels carry detergent and friction-modifier additives intended to keep injectors and valves clean over time. Supermarket fuel meets the standard's minimum additive requirement; branded standard fuel typically exceeds it, and branded premium fuel exceeds it substantially.
Whether that is worth paying for depends on the engine and how it is used. Independent testing has generally found no measurable economy benefit in an ordinary, well-maintained car. In direct-injection engines prone to deposits, or in a car that has been neglected, a period on a high-detergent fuel can help.
Why supermarkets price lower
Not because their fuel is cheaper to buy. Because fuel is not the product they are selling.
A supermarket forecourt exists to bring people to the store, where the margins are. Pricing fuel keenly is a customer-acquisition cost. A branded forecourt has no store behind it, so fuel has to pay for the site on its own.
That is why the gap is structural rather than temporary, and why it does not close when oil prices fall.
The practical answer
For most drivers in most cars, supermarket fuel is the sensible default, and the money saved is real and repeatable.
If your handbook specifies a higher-octane fuel, follow it — that is an engine requirement, not marketing. If your car is running roughly and has done a lot of short journeys, a few tanks of premium fuel is a cheap thing to try before spending money on diagnostics.
What is not worth doing is paying a branded premium out of vague worry. The standards exist precisely so that you do not have to.
Common questions
Is supermarket petrol bad for your car?
No. It meets the same BS EN 228 standard as every other petrol sold in the UK, and often comes from the same terminals. The difference is the additive package, not the base fuel.
Why is supermarket fuel cheaper?
Because supermarkets use fuel to attract customers to the store rather than to make money in its own right. A branded forecourt has no shop behind it, so the fuel has to cover the site.
Is premium fuel worth it?
Only if your handbook specifies a higher octane, or if you are trying to clear deposits in an engine that has been neglected. In an ordinary well-maintained car, testing has not found an economy gain that recovers the extra cost.