Sorry for the long thread title, but I feel this best describes how I'm feeling after driving home with 8 tires, some backpacks and a laundry hamper in the back of my car today.
I drove around an hour to my folks' place and averaged ~90mpg at ~50mph. On the return trip, the hatch was about 80% open to fit everything, and even using every trick I knew, I pulled in at my house with ~54mpg on the display. That's a
40% hit on fuel economy. I was barely able to maintain 50mph in 5th gear on level ground, and I doubt my average speed was as high, as I was frequently dropping into 3rd and even 2nd to get up hills.
For comparison, even pulling my aero-trailer with over 1000lbs of cargo I was still able to get just under ~60mpg at a higher speed of 58-65mph.
I must say, this really motivates me to make a boat tail. If I can get as large an effect in the other direction...