Just to set the scene.
I do about 700 miles a week from my home near Mansfield to work near Coventry in my 04 1.8 petrol....
4 miles town and bypass driving
54 miles motorway (M1 and M69)
11 miles dual carriageway and townish.
And the reverse for the way home
I usually get about 39 - 40 mpg using Supermarket fuel (Asda mainly, but sometime Morrisons), calculated at each fill - no trip computer on my car.
For the last tank full I have driven very economically, not revving over 2000 rpm before changing up, not going full throttle and cruising at 70mph on the speedo (which is 67mph on the TomTom).
I filled up yesterday night and calculated the fuel consumption to be 44.5 mpg, a massive 11% increase over my usual mpg. This is definitely statistically significant, I would say that less than 2.5% difference would be normal variation for my drive cycle. (485 miles and filled with 49.5 litres of normal unleaded).
My next fill will be Super Unleaded Petrol, just to see if their claim of better fuel economy is valid.
As I am going to India on Wednesday, I won't be doing this trial until my return, but I will post the results up when I do it. I intend to do the 'economy' drive first, and then maybe do a normal driving aswell.
I may even split out the economy drive on normal fuel too, to test the difference between my normal cruise speed, and the effects of not going WOT and over 2000 rpm, except in fifth gear of course.