Did the experiment this week with interesting results. My trip to work is 50 miles each way so I fill the tank every day (or day and a half if I am feeling lazy). I made the trip a couple of times crusing at an idicated speed of between 65 and 70 mph and when I filled up I calculated 45 mpg. I did the same trip a couple other times cruising at an indicated 75 to 80(+) mph and when I filled up, my mileage dropped to 35 mpg. Very interesting, I wouldn't have thought that 10 miles per hour would have made such a dramatic difference in my fuel economy. Well, at least now I know, and now I cruise at 70 mph
