Full guide
Compare the same grade first
A fair comparison starts with like for like: Petrol (E10) against Petrol (E10), or Diesel (B7) against Diesel (B7). Comparing a supermarket standard grade with a branded premium grade mixes together two decisions, where to buy and which product to buy.
The label at the pump tells you the grade. The vehicle handbook or filler flap tells you what the engine requires. If the manufacturer specifies standard fuel, paying for a more expensive product is a choice, not a basic compatibility requirement.
The main product difference is often the additive package
Retailers can use different detergent and performance additive packages while selling fuel of the same grade. Premium products may add a higher octane rating for petrol, a different cetane or detergent proposition for diesel, or marketing claims tied to a particular formulation.
That does not make every premium claim equally useful for every car. A performance engine designed around higher-octane petrol may benefit in a way that an ordinary engine does not. For a standard car, follow the manufacturer guidance and be cautious about assuming a higher price will repay itself through fuel economy.
Why supermarkets are often cheaper
Supermarkets sell large volumes and use forecourts as part of a wider retail visit. The CMA has repeatedly found that supermarkets tend to be among the lower-priced parts of the market. Local competition still decides the result on a particular day, so the nearest supermarket should be checked rather than presumed cheapest.
Use the current location comparison to see what each nearby forecourt is reporting. A retailer category is context, while the current pump price is evidence.
A sensible buying rule
Buy the grade your vehicle requires, compare current local prices, and treat brand or additive preferences as a separate choice. If you are testing whether a premium product changes economy or drivability, compare several full tanks under similar journeys. One unusually good or bad trip is not reliable evidence.
Convenience has value too. A small saving can disappear if the cheaper station needs a long detour, while a station on your normal route can be the better decision even when it is not ranked first.
Sources and further reading
Primary evidence for the important factual claims in this guide. External links open in a new tab.