I know you are not in favour of supermarket fuel, so I decided to check it out against my normal fuel which is Shell regular diesel.

I drive a Nissan Pathfinder and for my test I drove from Cheshire to Cornwall plus some local mileage - approximately 400 miles on Shell and the reverse trip on supermarket diesel, again about the same distance. I repeated this about 1 month later, but drove down on supermarket stuff and back on Shell. The difference on mpg between the two, was between 4 and 7%.

Thus on this very short trial, and as there was no more than 2p difference in pump price, supermarket diesel proved more expensive than Shell regular.