Now, here's a funny thing.
I was riding up the M6, M74 home from the Peaks yesterday when suddenly I got 20 miles less out of a tank of fuel than I would expect. On investigation there was nothing wrong with the Glee but my camping chair was bungeed on the back seat and it's canvas bag was frayed to shreds by the wind at both ends! A friend whom I had passed said it was flapping wildly in the wind - just the ends, about 3 or 4 inches each side.
It just goes to show the dramatic effect aerodynamics can have.