Cheapest supermarket fuel: our verdict from official price data

Britain's supermarkets have run a fuel price war for years, and drivers still ask a simple question: which one is actually the cheapest? Using official UK government fuel price data, updated through the day for thousands of forecourts, we compared every supermarket filling station in the feed. According to our data, supplied by government sources, one supermarket comes out on top for the most drivers: Tesco.

The winner: Tesco. Tesco does not own the single cheapest pump, and it is not quite the cheapest on average. It wins because it puts more consistently cheap forecourts within reach of more motorists than any rival. Of its 511 filling stations in the data, 233 price both unleaded and diesel in the cheaper half of the supermarket market, and 152 sit in the very cheapest quarter on both fuels at the latest reading. No other supermarket comes close on that measure, which is the one that matters most if you want a cheap fill wherever you happen to be.

How we picked the winner. We did not crown the supermarket with one headline-grabbing cheap station, because a single low price at one site does little for a driver fifty miles away. Our primary measure is the number of a supermarket's forecourts that are consistently cheap: priced in the cheaper half of all supermarket sites on both E10 unleaded and B7 diesel at the same moment. That rewards steady value across a whole network rather than one lucky pump. We then sense-checked the result against a stricter bar, the cheapest 25 per cent on both fuels, against the average price gap to the market, and against town-by-town comparisons.

The league, by consistently cheap forecourts: 1. Tesco: 233 of 511 sites 2. Sainsbury's: 168 of 315 sites 3. ASDA: 156 of 405 sites 4. Morrisons: 151 of 339 sites

Cheapest on average: Sainsbury's. Among the big four, Sainsbury's has the lowest typical prices, sitting around half a penny below the supermarket average on both unleaded and diesel. It also wins the most local head-to-heads, offering the cheapest supermarket unleaded in 103 towns where two or more supermarkets compete, just ahead of ASDA on 100.

The single cheapest pump: ASDA. The lowest supermarket price in the data is an ASDA site in Tilbury, Essex, at 142.9p for E10. That is the figure that tends to make headlines. It is worth knowing, though, that ASDA's average across all its sites is currently the highest of the big four, so its reputation for cheap fuel rests on its keenest forecourts rather than the whole estate.

Cheapest of all, with a catch: Costco. Costco's filling stations sit around 7p a litre below the average supermarket on both fuels, and almost every one falls in the cheapest quarter of the market. But Costco has only around 21 fuel sites in Britain and requires paid membership, so for most drivers it is not a realistic weekly option. We have kept it out of the main ranking and treated it as a special case.

Biggest network: Tesco. Tesco also runs the largest supermarket fuel estate in the data, with 511 forecourts, ahead of ASDA on 405, Morrisons on 339 and Sainsbury's on 315. The number of sites a supermarket reports is a good guide to how many forecourts it runs for the public, and it is part of why Tesco reaches the most motorists with a cheap price.

Who moves prices most: Morrisons. Supermarkets adjust pump prices constantly. Over the last fortnight Morrisons changed its prices most often, at about 1.3 times per site per day, just ahead of Sainsbury's. Tesco changed prices least often of the group. More frequent changes can mean a retailer is tracking wholesale costs more closely, up or down.

The bottom line. If you want the best chance of a cheap fill wherever you are, Tesco spreads consistently low prices across the most forecourts. If you want the lowest typical price and you have a choice locally, Sainsbury's is narrowly the keenest of the big four. And if you live near that one ASDA in Tilbury, you have the cheapest supermarket pump in the country on your doorstep.

How we calculated this. All figures are taken from official UK government fuel price data, published under the Open Government Licence and updated through the day. We used each forecourt's most recent reading, compared the supermarket brands against one another, and ranked on the number of forecourts consistently priced in the cheaper half of the supermarket market on both E10 unleaded and B7 diesel. Prices change frequently and vary by location, so the exact figures move over time; the picture of who offers consistent value across a network is more stable. This is general information, not advice, and is not affiliated with or endorsed by any supermarket.