Expert advice: is cheap supermarket fuel bad for my car’s engine?

cheap supermarket fuel
All major supermarkets such as Tesco, Asda, Sainsbury’s and Morrisons now sell fuel, frequently much cheaper than the big fuel brands (Picture iStock/jax10289)

We get a lot of queries from car owners about fuel quality. But the one that keeps on coming back is whether cheap supermarket fuel is as good as big-brand petrol and diesel. It’s an important question because there can be a significant difference in what it costs to fill up at a supermarket compared with at a fuel brand’s station.

We all want to save money where we can. Whether that’s with petrol or diesel that costs less, or apparently more expensive fuel that’s cheaper because it improves economy. But most importantly, we don’t want to do our cars any damage, so how good is supermarket fuel?

Cheap and cheerful?

When I was a lad, my mum used to say that the cheaper supermarket cornflakes she favoured were made in the same place as the big-name ones I wanted. It doesn’t matter whether that’s true or not. What matters is what we think. And because a product wears a badge (the supermarket’s) that isn’t associated with quality fuel, we don’t think it can be as good as the stuff made by people who specialise in it. But that isn’t necessarily the case…

cheap supermarket fuel
Major fuel brands such as Shell, BP, Total and Murco tweak fuel blends which they claim improves performance (Picture iStock/josefkubes)

Is all fuel the same?

The best answer to this is probably yes, and no. The cheaper supermarket fuel and the more expensive big fuel brand product on sale down the road could actually have come from the same refinery. Where they differ is that the big fuel brand will add its top-secret cocktail of chemicals. It will then spend millions on advertisements that claim its fuels increase performance, reduce exhaust emissions and even improve economy. And of course this is reflected in a slightly higher price.

Do big brand super fuels make a difference?

The maths is quite simple. If you do 10,000 miles a year and the fuel you buy improves your economy from 40 to 45 miles per gallon, you’ll be £90 in pocket, even if you’re paying 6 pence per litre more for your fuel. But will you really get 5mpg more? On most cars, that’s an improvement of between 10 and 15 per cent, which is a big ask. And we’ve already seen that when What Car? did some back-to-back tests, it found little difference between premium and regular fuel.

The car you drive

Some performance cars need a fuel that’s a higher-octane rating. This is because they operate at what’s known as a higher compression ratio. Essentially the fuel is compressed more before it’s ignited which releases more energy. Lower octane fuel will ignite too early to extract the necessary performance. If you’re in any doubt, check your car’s user manual. It should say if you need higher octane super unleaded petrol. And if you do, probably best to avoid supermarket fuel. But to be clear, putting a higher-octane rating fuel into a regular hatchback isn’t suddenly going to unleash supercar levels of performance.

What about bio fuel?

In order to reduce greenhouse gas emissions, all major fuel makers must now sell petrol with ethanol, a renewable non-fossil fuel in it. UK petrol has up to 5 per cent ethanol in it while diesel has up to 7 per cent of Fatty Acid Methyl Ester. But all new cars sold in the UK have had to be compatible with these since 2011.

There are legal standards

All fuel that’s sold in the UK must comply with legal standards set nationally and internationally. For example, currently the amount of ethanol in UK petrol is set at 5 per cent. But on the continent you can get petrol with up to 10 per cent ethanol. However, whether British Standards or European, the petrol or diesel you put in your car shouldn’t do it any damage. And that is the most important thing.

cheap supermarket fuel

Nick Reid is head of automotive technology for Green Flag and a fellow of the Institute of the Motor Industry

37 comments on “Expert advice: is cheap supermarket fuel bad for my car’s engine?

  1. Phillip 08/05/2019 9:36 AM

    You would make a good politician as that never give a straight answer to a simple question.

    • Jon 08/07/2019 7:14 PM

      Couldn’t have said it better myself!!

    • Paul Reynolds 08/07/2019 7:34 PM

      I agree waste of time

    • Chris 11/07/2019 7:05 AM

      I couldn’t agree more with Phillip. A simple ‘yes’ or ‘no’ with a reasoned explanation would suffice.

    • Mark Serlin 29/05/2020 7:06 AM

      The only simple things here are you lot, wanting a simpleton’s answer to a very complicated subject.

      • Dave 11/10/2020 10:48 PM

        Its not a complicated subject at all.The octane level of more expensive fuel burns more efficiently,gives off fewer particulates,and gives better mpg.

        • Mark 26/03/2021 7:52 AM

          Octane level will not improve your mpg, its the energy density of the fuel will do that for you. If you want to improve mpg use a premium gasoline containing friction modifier (FM) or a fuel with higher energy density (or both). If you take V Power or similar fuels they have a different cut from the refinery with higher density. The higher density of the fuel simply means it contains more molecules, more hydrocarbon molecules means more power. Many of the premium fuels also contain a FM that reduces internal friction in between the top piston ring and the cylinder.

          Octane ratings provide a specific rating of how well the fuel resists compression (ignition) and its important for the smooth running of gasoline vehicles. Engines with higher performance generally have higher compression ratios and need the higher ON/RON fuels however it will not significantly effect the mpg returned. Always run with the octane rating fuel recommended for your vehicle, it will run smoother.

          If you want to improve mpg, run a premium fuel containing FM. The FM on its own can save you up to 2%, anything more than that is smoke and mirrors. Another big factor for FE, especially with fuel injection and DISI engines is the cleanliness of the fuel injecctors themselves. Poor atomisation of fuel will significantly reduce combustion efficiency. Run fuel with keep clean claim and you will be fine in maintaining manufacturers as new performance.

          If you improve FE by 2% you reduce emissions CO2 by 2% its a lineral correlation, no magic involved

          If you are looking at Diesel, best you can hope is to maintain as new performance of vehicle with well additised fuel. FE on diesels can and will drop significantky due to fuel injector fouling, reduceing performamce, increasing emissions and reduces mpg. Basic diesel fuel in Europe at least should keep most injection systems clean. Cetane improver is sometimes added to premium diesel and if you inprove cetan number by 2 over basic EN590 requirments you may have smoother running in cold weather (again no imporvement in mpg).

          Most supermarkets sell a Diesel containing additives that improve performance above minimum legal EN590 requirments. Not I said performance and not fuel economy.. big difference and where fuel companies are careful about claims. Usually these (basic) additives improve keep clean performance but not clean up of the injection system or improve cetane numbers.

          The alcohol content of the E10 gasoline will have a lower energy density compared with the gasoline itself and therefor the fuel of vehicles running E10 will be lower. There is also a concern on vehicles produced in the 80′ and 90’s and earlier that the alcohold content can adversly effect the elastomers in the fuel system.

          Generally the big oil companies will add a better premium performance package (of additives) to both diesel and gasoline. Supermarkets generally tend to additise less. Premium hier density fuels with additives like V power I do not believe are availalbe at supermarkets and these obviously provide very best mpg and performance but are by far the highest cost.

          Remember mpg can be GREATLY influenced by how you drive. All fuel in Europe (and UK) is fine for your vehicles, not the same in all parts of the world!

  2. R D Sanders 08/05/2019 9:54 AM

    I carried out my own survey of Supermarket fuel versus normal Petrol Station fuel a few years ago. I compared 3 Supermarkets with 4 Premium brand fuels. Each comparison was over several thousand miles, so it took a long time. My car then was a Mercedes C200 petrol (Non turbo). In all cases the premium fuels returned higher MPG. Averaging everything out I calculated Supermarket fuels would need to be 5p per litre cheaper to break even with the Premium fuels. I guess its like almost every thing else – you get what you pay for.

  3. Gary 08/05/2019 4:01 PM

    Having worked in a Sainsbury PFS I can say that the fuel is from Greenergy in Grangemouth. It has the same octane rating as others but zero additives like the ones that clean injectors etc. Sainsbury’s also have water monitors on their tanks which alarms off at the presence of water.

    • Tom M 22/12/2019 1:31 PM

      No additives? Not true, they all have detergents as this is part of the standard. They’re just different, more generic additives than the stuff the majors add. The preponderance of the evidence is that it makes no difference at all.

  4. Barry Oakley 08/07/2019 7:28 PM

    Supermarkets don’t sell premium grade diesel.

  5. Sylvia Ravenhall 08/07/2019 8:45 PM

    Surely it also depends on what fuel outlets are convenient when you fill up? I know people who will drive miles to fill up on ‘cheaper ‘ supermarket fuel, which makes no sense at all. Conversely motorway service stations are to be avoided, if possible, as regardless of brand they always charge substantially higher prices.

  6. John Smith 08/07/2019 10:03 PM

    What about supermarket super grade of fuel? These are cheaper than premium brands and do give a higher mpg.

  7. Amjed Mahmood 08/07/2019 11:04 PM

    Hi I always put shell v power diesel in my minibus v good quality fuel good performance to engine more milage not much difference between supermarket fuel but quality is good

  8. Simon Calvert 09/07/2019 9:16 AM

    There was no mention of the levels of ethanol in the fuels. E10 fuels will give significantly less fuel economy than E5 or zero ethanol. Not to mention the damage that ethanol does to rubber seals on older cars and the deformation of plastic fuel tanks on motorcycles that were deigned to run on standard leaded fuels. Ethanol is also hydrophilic so cars that are not in constant use can end up with water in their fuel tanks. I think its generally good advice to avoid ‘e’s’ especially in classic cars or motorcycles or for that matter anything with a petrol motor that is not in constant use.

  9. L Barnett 09/07/2019 9:16 PM

    I’ve also done tests on my 2015 Ford Focus Petrol and found no significant differences in gas mileage between supermarket fuels and branded fuels

  10. Stanley Waterman 09/07/2019 9:55 PM

    I agree with the politician comment. This article is rather useless, no facts, stats, nothing- just a few paragraphs of verbiage. Bit silly! The information must definitely be known, to both the supermarkets and the other dealers but the public and Green Flag are be kept in the dark, why? However, I have a new Merc coming in two days time and will do a comparison for a bit of fun!

  11. Jeff Page 10/07/2019 8:28 AM

    I get less miles to the liter from supermarket fuels and lower performance, so I always go for brand name fuels

    • John Murphy 23/08/2019 4:52 PM

      I have an E 220 vid which is almost 11 years old( and I wouldn’t part with it for the world) and about 2 years ago I had it remapped by a reputable company to give me a balance of mpg and power. The result I can average 50 + mpg at motorway speeds on cruise and was told never to use supermarket fuel
      The car is running significantly better and is more responsive to standard.

  12. Dionysus 11/07/2019 9:14 AM

    I recall my local garage manager telling me that the best way is to run 4 out of every 5 tanksfuls on cheaper (supermarket?) fuel, then on the 5th fill up (and the emphasis was on filling up an almost empty tank) one get a premium brand so the additives do their job.

  13. Bob G 11/07/2019 3:20 PM

    Worked at a Morrisons PFS for 11 years until retirement. My ex-RN chum used to come in for petrol in his (then) Rolls Royce and never broke down afterwards. Higher premium may well be required in high-performance sports cars – but for the normal user, don’t get mentally “conned”. Go regular!

  14. Christopher Bleakley 11/07/2019 3:38 PM

    Tried my car on both no savings or performance issues…..

    • Richard 08/09/2020 10:48 PM

      It seems to be a long term thing. Engines do appear to last longer and take more stick with premium fuel. I’m a rep and on the road every day. Cars that the company furnish me with are Astras and the mechanics who service them say my cars are always as “sweet as a nut” compared to other reps vehicles. The only difference is the fuel I use. Others always fill-up with supermarket fuel. The mechanic reckons good petrol makes all the difference, especially if anyone does high mileages.

  15. Peter whiitefoot 04/08/2019 11:41 AM

    Unfortunately Shell’ V Power super unleaded is expensive where I live near Mansfield notts but I would definitely still fill up with their independent petrol station as against Sainsburys super unleaded which is much cheaper. Reason? I have done my comparisons and my Mercedes 180 kompressor reacts far better to sudden increased speed needed, does more miles to the gallon and runs smoother. I really couldn’t tell you whether it is down to a superb high compression engine that the Mercedes gives you but I can tell you it works for me . I also tried Texaco super unleaded ….not as good and was more expensive than Shell !

  16. John roe 15/01/2020 6:43 PM

    I have driven my Alfa Romeo Gt, owned from new as a factory order now approaching 180k miles. I have always used Tesco fuel in it. It has never let me down. The car is 15 years old and oil consumption is negligible.

  17. Prince 26/06/2020 12:32 PM

    I was on shell premium on my E90 after i reconditioned my engine.. i got 32mpg on my long journey.. average local trip gave me 28/29Mpg. Once i tried with saintsbury premium petrol and on a long trip i got average of 41mpg.

    • David A Dumville 21/01/2021 4:46 PM

      A 25% increase in mpg. If that was true we wouldn’t be having this conversation because everybody would know to use supermarket fuel

  18. Mick Harris 12/07/2020 8:19 AM

    really Prince…that’s impossible…you were obviously driving much more economically…or working it out wrong.

  19. Gavin 06/09/2020 12:00 AM

    I have used all supermarket diesel fuels in my Lexus, it was a case of the nearest filling station at the time. Convenience !!!!!!
    But then a mechanic said my car was choked up with carbon, diesel particulate filter was getting clogged and performance wasn’t the best. I made lots of changes not just my fuel. Firstly oil, a good top quality oil and make sure it’s the correct grade for your car. Better quality filters, air, oil and fuel. Then used shell v power diesel and run additional cleaners and fuel lubricants through the system. Always add 25ml of a cetane booster to every tank of fuel now. Results after 18 months of running the car after the changes… smooth, very quiet engine, passes MOT emissions first time, greater MPG, no smoke from exhaust ever even on cold morning starts, acceleration is fantastic. These things all done together really do make a difference. Can’t say it’s just the good fuel, or the correct oil but look after your car and in the long run it’s cheaper as it rarely needs the garage or a mechanic these days. That’s my proven results.

  20. Richard 08/09/2020 10:41 PM

    Supermarket petrol is okay and it does the job. However, anyone who wants to protect their engine to the best of their ability would be be better filling-up with big brand fuel. I’ve also found that even the most “weedy” cars run better on Shell or BP. They run cooler for some reason and mpg is a little better. Some of the additives incorporated in Shell , BP and other main suppliers really does help in protecting the engine. If someone changes their car every 3 years or does low mileage I suppose it doesn’t really matter. That said…I travel 12000 miles a year and each time I’ve had no choice but to fill-up with supermarket fuel, I have noticed a difference. The cars tended to run less smoothly. Pinking sometimes occurred and I there was a reduction in power. I’m a rep and drive Vauxhall Astras.

  21. G fele 26/10/2020 3:29 PM

    Premium fuel is better than supermarket fuel, but is it 4p to 5p a litre Better ?.

    • Peter dB 12/03/2021 6:38 PM

      In my case, I use up to 50l every 2 weeks. If I pay 5p more for branded diesel (1.25/l at the mo), it will be £65 per year more expensive than supermarket diesel at a total fuel cost of £1625. Add other costs for driving and servicing a car, then it’s really not that much more for peace of mind and/or perception of improved mpg and performance.
      Having said that, for those £65 I could drive the first 2 weeks of the next year.

  22. Peter dB 12/03/2021 6:39 PM

    In my case, I use up to 50l every 2 weeks. If I pay 5p more for branded diesel (1.25/l at the mo), it will be £65 per year more expensive than supermarket diesel at a total fuel cost of £1625. Add other costs for driving and servicing a car, then it’s really not that much more for peace of mind and/or perception of improved mpg and performance.
    Having said that, for those £65 I could drive the first 2 weeks of the next year.

  23. Darren 07/08/2021 6:58 AM

    Can you just answer the question ? .why don’t you go for the prime minister’s job

  24. Ken 07/09/2022 9:50 AM

    I have had this discussion with friends and some say supermarket diesel is just as good as the onboard computer says so. My onboard computer says at times that I get 100+ mpg. The only way I would get that would be over a cliff with the wind behind me. The only way to determine your actual mpg is by filling your tank completely and dividing your miles done by the fuel added. I find that branded fuel gives me better consumption and value, however, everyone to his own.

  25. Peter Gartrell 02/05/2023 7:49 PM

    Loved my previous Alphas, but your reliability is Guinness Book of Records for the marque!

  26. Dave the detectorist 13/01/2024 10:28 AM

    Switched to supermarket petrol about a year ago in my 05 1.8 Nissan Primera and noticed a significant drop in mpg.I tried both Tesco and Sainsbury’s petrol both 95 and 99octane with no improvement.

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