Would they coast if the road was perfectly level?
I don’t buy all this theoretical testing. Why isn’t the mileage figure derived by having an expert driver drive a car in real world conditions to see what can be realistically achieved? By the way I am convinced that the single greatest thing to be done to increase real world MPG would be to junk the cruise control and teach people to drive for best mileage. The meme that people should always use cruise control on the highway to improve mileage is absurd. I have proven over and over that I can achieve well over the EPA mileage figure WITHOUT using cruise control but as soon as I return to using it my mileage drops rapidly. The only exception is if you are driving on very flat roads and then the cruise control does ALMOST as well as I can do without it. Anyone who tries to use the cruise control on very hilly roads is simply fooling himself.
To coast on a perfectly level road, you just have to lift the back end of the vehicle up...then you are always going down hill. Physics is easy.