You did not go from 39 to 47
You did not go from 39 to 47 MPG because the battery is lighter and more energy dense, while yes lighter does help 40 to 60 pounds is a few bags of groceries, otherwise you would see massive losses when driving with another person in the car. No, you got a huge boost because your battery pack was old and couldn't hold as much charge. This is something that may be fixable too if it's just due to some dead cells