[stumped]

I have a 2005 Prius. We were getting over 50mpg. I bought new tire (37,500) and the millage dropped to maybe 48 but started to come back up as the tires wore in. Just recently (last couple of months) the millage is around 47. No matter how hard I try I can't get back to 50. Also within the last couple of months I have noticed that all of the gas pumps in town say "contains less than 10% ethanol". I know ethanol has less energy per gallon than gasoline, but do you think the ethanol could be hurting my millage that much? Is it possible that maybe the battery is not working as well with age and millage (75,000)?

Thanks for the input ... Michael