Hi Guys, Can I tell you a story.
Back in 2003 I purchased a brand new Landrover Freelander and after driving it for a couple of months, I was of the opinion that it was not performing as it it should, in terms of general smoothness and Mpg etc. I spent 44 years at Landrover so I know a bit about the vehicles.
When I spoke to my dealer about my concerns, his first question was, "What fuel are you using?" to which I replied 'Supermarket'. He then said, "Try a couple of tankfuls of branded fuel, Shell, BP or whatever and then come and see me"
I did as he suggested and I have to report, there was a significant difference, so much so that the increase in Mpg more than covered the increase in fuel cost.
Now, I know of course I are talking diesel rather than petrol and is was 16 years ago, but I maintain there is a similar difference in petrol as well, certainly my Jazz runs better on branded fuel
Petoe48 may be right regarding Asda tankers and the difference in performance may well be down to the additives, I just don't know, but I have never used cheap fuel since.