Well, I would say the sweet spot is around 45 for longer trips. When I'm not doing my monster commute on the epressways, mostly on the weekends, I've found that going 47
MPH, which allows me to not only hit all the lights in my area green, but with enough time for those stopped to get up to speed, allowing me to go over 25 miles without varying speed at all, on a nice flat road, nets me around 50 MPGs in favorable conditions (No strong headwinds, freezing weather, etc.), and eliminates all start and stopping for the majority of my trip, just when I reach my destination, and when I leave. So really, I think it depends on your trip and where you are.
If I went slower, I'd probably get better MPGs for the cruise, but I'd have to start and stop at the lights. I tried going 42 once, and ended up stopping four times, and starting four extra times.