Hi Gary, I am advised that the branded fuel, Shell, Esso, BP etc has various additives that suit performance engines whereas supermarket fuel has not. Some drivers put various additives in the fuel themselves. I am assuming at your car is powered by the 2.7 diesel as is mine in which I got 53 mpg on a mixed roads drive of almost 100 miles in which the motorway was speed limited to 50 mph. The return journey showed 49 mph when I was able to give the car a bit more speed. Regards, Peter.