After using Tesco 99 for months I have now swapped over to V Power. I have noticed a slight increase in mpg over the tesco fuel.
I have a standard '05 gti. My run to work is 10 miles and would get 34 or 35 mpg with tesco 99 and now I am easily getting 37, sometimes 38.
I think the car feels a tad quicker too.
I always have heard that supermarket fuel is of a lower quality than that of the branded companies like shell or bp. I was told that when the big tanker ships are quality checked, if the fuel isnt quite right, like the s.g., it is sold at a lower price to the likes of the supermarkets. The fuel is still good, but of a lesser quality. Whether this is entirely true I' not sure.