In my own experience, yes, but not by a factor that would actually be worth the extra expense. Standard Shell diesel gets me reliably 2mpg more than Asda fuel, and V-Power about an extra 1.5mpg on top of that - neither of which justify the extra cost alone.
The more important thing, in theory, is that branded fuel has additives that can improve engine health and so on. But nobody knows what is in there, really, and there's nothing to say Asda or Tesco don't have the exact same additives.
There have been a few tests done, and most seem to suggest there's almost no real world difference, particularly for modern, well maintained cars. If you have an old car with a complicated engine (like my 16 year old V8 range rover), then it might help a bit more, but all you're paying for is the equivalent of a bottle of Redex you could just buy every couple of months yourself anyway.
The most important thing to consider - it's not like Tesco runs a refinery, the fuel they buy will be the same fuel Shell or BP use, it all comes down to the proprietary additives, that you will never find out what they put in or how much of. My advice is to just try it and see what happens, as some engines will respond more than others to fuel changes.
There's also 99 octane fuel like VPower and BP Ultimate, which has tangible benefits, but only really for high performance cars.
Tesco do sell 99 RON premium at some forecourts. All British petrol is basically fine because of our emissions standards - there's no point mandating efficient engines with low emissions if people are allowed to run them on crap fuel. American diesel is full of sulphur and a lot of the fuel in South America and South East Asia has bits of carbon floating in it.
Any reasonably modern car will run fine on cheapo unleaded, because the ECU will detect and prevent knock. You might lose a bit of performance and/or efficiency, but it won't be damaged. The only risk of disaster comes with turbocharged cars from the late 80s/early 90s or anything that has been "chipped" by someone called Darren. As >>4527 says, any improvement in efficiency from premium fuel is outweighed by the higher cost.
Maybe the special detergents and additives in premium fuel improves engine lifespan, but I haven't seen any convincing evidence. Buy cheapo fuel and spend the money you've saved on regular oil changes, which really does make a big difference.
Tesco's 99 is good and is what I run my silly cars on. Factory tuned sports cars often seem to be tuned to run best on Shell V-Power, but you have to be on the nerdiest side of car nerd before you're thinking about those sorts of marginal gains, and if you're that much of a mentalist you'd just tune it for Tesco 99 if you wanted to save a few pennies.
In general, for an average car you'll save about three quid per tank. A bottle of fuel system cleaner/fuel additive is about three quid too, and using one of those every single time you fill the tank is overkill, so unless shell's additives are fucking fantastic, then Asda fuel plus some sort of shop bought additive like redex every three fills would almost certainly be the same, if not better, than branded fuel.
As said, with my old diesel v8, I can tell straight away that it likes to run on shell - you can hear the difference, and feel it, and I don't think it's placebo at all, as two of my car nerd friends have commented unprompted before that my car sounded lumpier, and this was when I had to put Tesco in it.
But, of course, other than lexuslad, I'm willing to bet nobody else here is running a finicky car from a decade or two ago, so I would absolutely not recommend using branded fuel for your car. I would also say that the difference in price is not really huge, so I wouldn't worry about it. The most cost effective petrol is likely the one that is closest to you, is at the supermarket you already shop at, or directly on your route to work etc.
Also, Shell used to actually balance out, when they did loyalty points - you would actually end up with cheaper fuel than anywhere else if you used them exclusively. They've changed the system now to one where you get about two quid off every ten visits, which is just exponentially worse a deal.
It was a Mythbusters episode, so a massive pinch of salt is to be taken but I remember they did one where they found little difference between brands. The biggest deciding factor in MPG was the driver's mood that day.
I do wonder, though. My LS400 (which never came with a turbo) is recommended premium fuel only, but is also converted to LPG. LPG doesn't come in variants, but has a RON of about 110; I wonder what the ECU makes of it. I know the LPG computer is overriding it, but still, it's the equivalent of slowly-delivered race fuel. Switching between unleaded and LPG is done every time the car is started (it starts on unleaded then switches to LPG once the engine is warm enough), and can be switched between simultaneously at the press of a button, with no knocks, pings, or anything.
Most 90s jap cars will have 'premium fuel only' on the fuel cap or in the manual, but this was more to do with local and yank markets than us - Japanese 'regular' petrol is only 90 RON.
I don't know much of anything about LPG, I'm having a hard time getting my head around how you would program for it in an ECU. I wonder if the relatively basic 90s ones would even understand the fuelling rates.
>>4533 I imagined that'd be the case, though the US octane scale is slightly different to ours - their '87' is our 95, meaning that it was still asking for our 98/100. Either way, I put the good shit in anyway since I only ever run it on unleaded if I really want to put my foot down or can't find LPG close. I fill it up at the Costco where it's the same price as normal fuel everywhere else.
It seems to understand fuel delivery somewhat as the MPG figures on the dash seem a bit lower when I'm on LPG, but that might just be confirmation bias as I'm not always staring at it. It does get confused though, since it's usually only running on LPG under heavy drain conditions (startup and heavy acceleration), it thinks half a tank of fuel will only get me about 20 miles.
Supermarkets sell the product as cheap as possible to get people into the shop itself.
On the other hand, Shell/BP/Esso etc. have wafer-thin margins in urban areas and so whilst their prices are higher they make the product itself as low-quality as they can get away with because they need every last penny to turn a profit.
The premium fuels definitely have extra additives which might clean your engine, and the higher octane etc. might give better economy depending on your engine, but I don't believe that there is any difference at all between regular fuels.
However it is worth remembering that as fuel pumps measure volume, during a cold snap you can cram just a little bit more fuel by weight into the tank for the same price, but also I have heard that the refiners tweak the blends very slightly through the seasons to counteract this, as well as to control evaporation.
Also,
don't bother with redex, nowadays it's just kerosene with some red dye in since people get a bit antsy about some of the spicier chemicals that could be in there.
Get some of the cleaners sold by Forte instead, it's sold as a "trade use only" product, but it's easy enough to find online, and it gets well recommended by a lot of people in the trade who claim it can get cars through an MOT after failing emission checks.
