One of my first ecodriving techniques with my current car, was to ever so gently reduce the throttle. It kept up the speed (sometimes it'd lose 1 or at worst 2 mph), but fuel consumption dropped significantly, and well beyond what you'd expect from a 1 or 2 mph drop in speed.
So it made the same power on less fuel, i.e. by leaning it.
It was rather tricky to reduce the throttle, as overdoing it meant the speed dropped like a stone, and any bump could change the position of my right foot, often ruining the whole thing.
As far as I can see, the cruise control seems to do something similar, but far more consistently, and with a lot less effort on my part
