Here is a scientific explanation from a former member who is not longer with us;
This topic reappears every so often, but in essence I'll quote my answer about diesel fuel - and petrol is pretty much the same story:
As the UK Representative to CEN WG9 (European standards committee for all motor vehicle fuels) all pump fuel in Europe is produced to meet the relevant EN standard - which you will see written on the pump body.
The major difference between the supermarket fuels and the branded fuels is the exact nature of the additive pack added to the fuel when it leaves the refinery - common rail pump lubricants, injector cleaners, etc.
Synthesised Diesel
The exception to this is the new "synthesised" diesel fuels, such as BP Ultimate (actually researched by Aral in Germany), Shell V-Power (may be called something else in the UK), and Total Excellium. These fuels are manufactured in the refinery by joining simple petroleum hydrocarbons into an exact diesel fuel - you'll need some experience of university level Chemistry to follow what they do - so just accept that they are better - higher cetane rating, better additive pack, etc.
Normal Diesel
This is a straight distillation fraction from crude oil, produced by the nearest refinery to the fuel depot - so for instance, diesel refined by Shell may be sold by any of the other retailers close to that Shell refinery. The major difference is the additive pack - which is brand specific - and any specification difference imposed on the refinery by the other retailers - and the addition of bio-diesel.
Bio-diesel
Another one of our EN committees, pump bio-diesel is a blend of normal refinery diesel (95%) and (5%) of pure bio-diesel. This is an EN standard and all EU countries will be (or are) selling this bio-diesel as a direct replacement for normal diesel. All the car manufacturers have accepted this 95:5 blended fuel, and we are working on specifications for a 90:10 blend.
Pure bio-diesel is manufactured by mixing and heating vegetable oil with methanol (methyl alcohol) and sodium hydroxide (caustic soda). This splits the vegetable oil into glycerine and fatty acids, which immediately reacts with the methanol to form a "fatty acid methyl ester" or FAME (bio-diesel).
FAME is perfectly fine as a diesel fuel, it has very good lubricating effects - and replaces the lubricant additive in a normal diesel fuel - except that it is not as stable as normal diesel - it tends to go "sour" or "rancid", which is why the car manufacturers don't like you to run a car on pure bio-diesel. Not a problem as a 95:5 blend.
Vegetable Oil
Reasonably OK for an old (pre 1995) diesel engined car - except that the car won't start on cold vegetable oil, but once the engine is running it's OK.
Other problems are that vegetable oil quickly turns in a gummy glop (like the linseed oil that artists use) and the car's fuel system and that the engine needs a lot more maintenance - blocked injectors, gummy residue in pumps and cylinders, etc.
After market additives
After market products like Millers improve the cetane rating of standard diesel, but only when the engine is cold - interestingly it doesn't help a hot engine - so cold starting is usually quieter but no difference to a hot engine - and Millers does provide good, additional pump lubricity.
Self Tuning Engine Computer
So, if your diesel engined car "self tunes" then try Shell V-Power, BP Ultimate, Total Excellium - I see between 3% and 5% improvement in fuel consumption. My Mercedes C270 returned 52.8 mpg driving from Rotterdam to Wendover yesterday - on Dutch Shell V-Power - 320 miles at speeds of around 60, 70 and 80 mph depending on the country / road speed limit - using the electronic speed limiter. If not, stick with the regular diesel fuel, and add Redex or Millers at the recommended dosage level - adding more won't improve the performance. And don't be dismissive of supermarket fuel, it can and often is identical to branded fuel.
And if you add Redex or Millers or switch to a branded fuel, then any change to the cleanliness of the injectors won't show itself for quite a few hundred miles, but pump lubrication and maybe a higher cetane rating will show an effect much sooner - depends how much old fuel was in the tank and fuel lines - and how much the old fuel dilutes the action of the new fuel.
So is there a difference between supermarket and branded fuel ? - there can be - but often there isn't.
How can you tell if one fuel is better than another?
To compare the fuel consumption, you need to exactly reproduce two journeys - exactly the same speed, exactly the same acceleration and braking, and under identical conditions:
Atmospheric Pressure
A one percent change in air pressure has an identical effect on power and torque - so driving on days with high pressure makes the engine generate more power
Temperature
Driving on hotter days reduces engine power.
Humidity and Rain
Driving on days when it is humid or raining significantly improves engine power - water injection is used in truck racing and sucking in damp air has a similar effect in increasing power.
These produce percentage level effects on mpg - making it difficult for the driver to make comparisons. Driving on a cold, damp day may see an improvement of 3% or more compared to a hot, dry day.
Even more important are the effects of different traffic levels and the inability of drivers to EXACTLY reproduce a journey on UK roads, for instance:
Speed
A 1 mph increase in speed (say 60 instead of 59) will make about a 2% difference in fuel consumption - rolling friction and wind resistance increase exponentially - on top of the extra fuel need to spin the engine that bit faster.
Unless you drive everywhere using an electronic speed limiter or digital cruise control then it's impossible for most drivers to reproduce even a constant speed.
Acceleration and Braking
Big percentage differences here - and unless you are driving on an empty test track - the effects of other traffic, let alone how you drive the car, have effects at least as large as the difference attributed to different fuels.
Summary
There is a reproducible improvement of between 3% and 5% by buying the synthesised diesel fuels - BP Ultimate, Shell V-Power, Total Excellium, etc - compared to the standard branded diesel fuels.
There are smaller differences between supermarket and standard branded fuels - sometimes they are physically the same fuel - sometimes they differ only by the additive (cleaner) pack - and sometimes they are different.
Day to day variations in the weather, driver reproducibility and traffic make it very difficult for drivers to reproduce journeys.
Comparing two fuels
If you do want to make a comparison, drive your car until the fuel tank is almost empty, then fill the tank and after you have driven 200 miles (any old fuel should have been used up), drive at a fixed speed on a motorway for 10 miles and record the fuel consumption.
Then the next time you fill up, repeat the exercise with a different brand of fuel - but remember to test on exactly the same section of motorway and on a similar day.
Checking your fuel consumption over normal driving, in stop start traffic, over a period of weeks - just tells you that you have had to driven differently.
And don't forget the placebo effect. Robert
Facebook
Twitter