Right, I know alot of you are saying its Bull-****
But, we have a Renault Clio Dynamique (06 plate) 1.6 litre + a Smart For Two 0.8 litre (05 plate) and they both seem to be affected when we use supermarket petrol, Well Morrisons petrol to be precise.
The renault clio, when we fill up at esso averages 45 - 48 MP/G and generally feels relatively responsive for a car of its size + engine size, when we've used morrisons fuel, we're lucky to get 35 - 37 MP/G, and there seems to be almost a lag from when the accelerator is pressed to when the car actually does anything, this has been tried a couple of times switching fuels.
On the smart car, using esso fuel we can achieve around 52 - 60 MP/G or about 250 miles per tank (20 litres) On the morrisons fuel it drops sharply to 35 - 50 MPG and we achieve about 150 miles per tank of petrol. and the car seems much less responsive.
Now i'm not saying this is the case with all but for the extra bit of money you usually have to pay verses supermarket from my experience is actually worth it.
Plus, we actually pay less for esso then we do for morrisons, £1.25 per litre V £1.32 last time we filled, but I think thats an unusual case because the esso garage is on a main road through our village with morrisons being the next one down so they under-cut to stop people filling there as much when doing shopping
