Most people misunderstand the way the marketing men (who are not scientists) come up with "defendable" claims.

The vast majority of any additive is a detergent/cleaning agent, and this is how they justify the "up to x% more mpg" claim.

First, take a car thats been run for a considerable time on basic fuels that have little or no cleaning additive and/or high levels of gummy contaminants. Make sure its had a difficult life such as short round town trips where its not had enough time to get up to temperature or develop high power/revs. This ensures that the fuel injectors/jets and other engine components get all gummed up with carbon and that brown sticky stuff thats incredibly difficult to shift if you've ever tried.

The mpg is poor because the injector spray patterns are poor.

Then try your new superfuel or add your additive and after a while - voila, a miraculous increase in mpg.......brought about by the injectors getting cleaned and the spray patterns restored.

Its a valid claim.....for some people. It depends on the design of the engine/fuel system, driving patterns and fuels used. For others, particularly high speed, high mileage motorway mearchants, the benefits will be neglible.

Keep drinking the Kool Aid. The additives have a negligible effect on how the fuel is burned. Emissions are based solely on how the fuel is burned. When I had the cylinder head off the Renault, I examined the combustion chambers and found them to be exceptionally clean-as were the intake valves. Where were all these deposits?

This was after 10 years of running the cheapest fuel available-and total negelect of the fuel injection system...

The marketing men use pseudo science and pretty pictures of explosions to convince a gullible audience that a certain brand of fuel will burn hotter for longer and all that carp. It's total nonsense.

Have you ever seen any of these scientific tests in real life or are you speculating, I suspect the latter.

I have actually witnessed the testing that takes place and it's nothing like what you sceptically described.

Firstly the lab technicians/scientists spend huge amounts of time and effort (at great expense) trialling numerous additives packages, it's only once they have completely rigorous testing that a fuel additive package is released for public usage. The companies concerned face potentially £millions of liability claims if they get it wrong.

When they test a particular additive package they have to ensure there is absolutely no trace of any previous additive package within the fuel system. A complete drain and flush out of the entire fuel system is carried out prior to any given test procedure, otherwise the test results are contaminated and the test would be deemed null are void.

The additive package used in Shell FuelSave is designed to ignite as uniform a combustion burn pattern as possible to ensure as much energy is extracted as possible from any given injection of fuel within the limits of any given engine design. Some engines are inherently more effective at translating combustion energy into motion and mpg, engine cleaning similarly applies (i.e. some engine designs are inherently more effective at staying clean than other designs.

Just taking the cylinder head off and observing if it looks fairly clean doesn't show what else is going on in the rest of the engine. A petrol engine with indirect injectors that are out of the way of a direct combustion explosion may have relatively clean injectors, whereas an engine with direct injectors are more vulnerable to carbon deposits as a result of their proximity within the actual direct combustion explosion cycle.

In addition, for example, in a diesel engine the cylinder head may look clean but it doesn't necessarily follow the rest of the engine system is clean, the MAF and EGR systems could be filthy which would directly have a massive negative impact on fuel economy and mpg.

Coming back to your original query. The mpg difference is negligible, too many variables involved to nail it down to anything in particular, weather temperature changes alone of say 5c will noticeably impact mpg up or down, wet roads, head winds all impact on mpg, slightly differing driving style, differing traffic conditions, the variables are many.

The fuel companies have to repeat their tests over and over and obtain the same results again and again, they do so in control conditions as it's the only way of providing predictability and reliability. If they change one parameter, say one of the chemicals in their additive package, they do get a different result.