I kept noticing that my mileage was going up and down from gas station to station. I asked my stepdad who runs a BP ( Amoco ) here in North Carolina about this. In NC the stations can mix up to 10% ethanol with the gas and they do not have to post this to the customers.
I ran into someone else who let another person drive their car and they decided not to buy one due to the gas mileage. When he heard of this he took the car into MB to see why he had a mpg drop they said to try a Shell gas station and his mileage went back up. The percentage of ethanol varies from station to station and sometime the same brand of station may or may not have it.