Not to detract from the talk about concrete slabs but I was thinking about the original topic of this thread when I drove back to work yesterday.
600km trip approximately 1/2 proper pavement and about 1/2 "thin membrane surface road" Both parts have very similar surfaces but I have always had better fuel economy in the south with the proper pavement and I decided to see what kind of difference there really was.
I used average FE from 30km sections along my drive that had no traffic. speeds were 90, 100, 110, 95, and 105km/h in that order for both road surfaces. Wind was very light a 5km/h from the west, travel was going north / north-east.
I was sort of surprised at how much of a difference there was, about 8%