Yeah, that would work, but it takes a lot of time. With enough aero, you don't have to slow down.
I took mpg vs mph data, multiplied by $6/gal and $6/hr, and added up my cost per mile at various speeds. I find that I should drive my Insight as fast as I can get away with driving to conserve time any fuel.
Maybe if I were getting less than 60mpg at those speeds it would be a different story. But anyone who can get 45mpg at 75mph, should do so. When I see a Prius zip past me at 80mph, I nod and think, "yeah, that makes sense".