In case it hasn't been mentioned, your driving behavior can also dramatically reduce your mpg rating. No jack rabbit starts,
being miserly with acceleration, keeping speed down on highway etc.
I love the tail you built out of cardboard and duct tape, very nice. I'm impressed, really! I'm inspired, being a duct tape afficionado myself.
On a side note, your end result with the tail on the hatchback closely mimmicks what some sedans look like.