That is kind of a funny post, I just took a trip nearly 400 miles round trip. Started off going 70mph because I was trying to get their at a fair time, by about half way the low MPG numbers were driving me nuts (like 36 on the gauge, not calibrated, normally i'm 40-50), so I dropped it down to 65 and gained atleast 1.5mpg average by the time I got there. On the way back I had a good 200 pounds of extra weight in the back, and I drove 60mph all the way. MPG meter read 41.5 when I got to my house
. I normally drive 45mph, in a sweat spot that my gauge says I'm getting ~50+mpg all the time.
55 on the highway is kind of hard to do, what I like to do is find a semi that is going around 60 and follow it. Not tail gating or anything, just follow. #1 you do get a little bonus in mpg if your not too far back, and #2 the people behind you can see the semi and knows you are going slower, at least if the drivers have half a brain and are not sleeping at the wheel.
Overall I only had 2 or 3 tailgaters on the trip and it was a really nice relaxing drive. I was picking up parts, so the drive was my vacation lol.