My A2 perhaps runs a bit quieter on Shell Extra diesel but I have no idea about MPG. All Shell stations in the North West have been selling it since early in the year, but 2-3 months ago Extra was not available in Oxford. Maybe it has spread further since then.
Hi!
Just a word about premium fuels...
I've being used the Shell Diesel Extra and BP Ultimate Diesel since those arrived to the market (aroung 1 year ago).
I haven't notived any improvement on mpg, but the engine runs quieter and smoother. The rpm climb is much more smooth, actually giving an idea that the car runs faster. The top speed is unchanged, but 0-60 is slightly better.
I will not return to "normal" diesel, since this qualities (although smaller) creates a much smoother and quieter ride.