A bit of a bump of an old thread this, but I've been away.
Absolute mpg figures are difficult to work out as precise fuel measurements are difficult and there are inbuilt inaccuracies in mileometers (remember speed dials under read slightly -different tyre sizes, manufacturer not wanting to be sued when you are caught speeding). Whats more interesting is the relative mpg. I track my mpg so I can see when any variations start to creep in - this can suggest a whole range of things from fuel leaks, stuck EGR valves, wheel tracking out etc. So if your mpg suddenly drops for a reason check it out!
As for the claims of the fuel manufacturers these should treated with as much caution as vehicle manufacturer mpg figures - carefully. As vehicle manufacturers have raised the mpgs of cars by reducing weight (removal of real spare wheels is one way), carefully looking at test wording for get outs, so fuel manufacturers can do the same. Under laboratory type conditions the improvements in mpg are possible - but in the real world traffic and driving style as well as load, vehicle condition are all more significant. The British magazine Which? (Consumers Association) found the claims for improved mpg unlikely to be achieved in the real world.
However, that is not to say these fuels aren't worth using - but that depends on how you use your vehicle. Diesels with all the anti pollution gear fitted like DPFs need long runs for them to work properly - or else you get DPF failures, stuck EGR valves etc. So using the fuel giants fuels with all the cleaning additives may be helpful.
On the other hand I regularly do 100 miles a day on motorways at speed and I nearly only ever use Asda's finest. With 87000 miles on the clock on my car I have had no issues. I have also driven a Landrover throughout the Northern Sahara and Morocco where fuel quality was..interesting (it was watered down - heavily). This was fine but the moment I hit France I went round to the local Shell or BP station (I forget which) and filled up with the premium diesel product to clean out the rubbish in the fuel system. I know of one case where an LR driver filled up on cheap Libyan diesel many years ago (15p a litre!) and was somewhat surprised when it froze solid when he hit the mountains of Italy in winter!
So what do I do? Once every so often I give my 87000 mile cars treat - a tank full of premium diesel from one of the fuel majors. No difference in mpg at 80mph, but I hope its cleaning out the rubbish from the fuel system!
As for my VW camper - Asda's finest! But its less than a year old and only does BIG runs!