On the side of the ecu there is a fuel quality switch. It is a 5 position switch. This gives your car more unriched fuel ratio. I think the position for US cars is #1. If your running a chip you are best to turn it to postion zero. This is default and allows the ecu to take inputs (afm-o2sensor etc.) to determain appropiate fuel ration. Which is to say it will adjust the ratio everytime the ecu samples and gets inputs. Because gas station fuel is never exactly the same (not held to race car standards) all the time there are slight differences. The stations tanks have influence as well. The tank may be taking moisture or have a lot of contamination. The filters at the pump help eliminate these but they are still there. This is where the switch comes in handy. The switch was put there to compensate for different fuel quality from country to country, hence the name.