Does anybody know if a weak or failed accessory battery affect mpg? I noticed my mpg suddenly dropped from around 46 to around 41 over the past several months. I have owned my ford fusion hybrid 2013 for 3 years in florida and have the same driving patterns, but could not get an answer from the ford dealers. They blamed it on more ethanol in the gas. I researched that and could not find anything on that. I was in for an oil change and asked the dealer to check the accessory battery because my car would not stay in accessory mode listening to the radio before turning off. They found a bad cell in the battery and had it replaced and low and behold, my mpg has jumped back up.