is supermarket fuel bad?
 

i have heard a lot of people say that supermarket fuel is worse than branded fuel. does anyone have any proof of this?

I don't know how it works across the pond, but in North America, the supermarkets will buy the gas from whichever company can provide it for the cheapest at the time. One month it could be supplied by exxon, the next month, shell.

The only possible difference in quality is that in order to have the lowest price when selling to the supermarket, some suppliers might not include the same additives that they do for the branded gas.

I have found that in my '88 Escort I see a considerable difference in some major brands. Texaco or Shell will yield about 10-15% better FE than BP or Citgo. I can also buy gas at Walmart or Sam's Club and get about the same FE I get with Texaco or Shell. In my '97 Escort wagon brand doesn't have much if any impact on FE.