For coastdown testing, you only need enough road to get up to speed, then coast down. Steady state cruise mpg metering (driving in a loop while watching the scangauge) isn't as accurate, because it includes variations in engine and drivetrain efficiency, and inaccuracies in measuring fuel consumption. You can use it to make an mph vs mpg graph, but I wouldn't use it to compare mods.
I've been looking for a good piece of road for coastdown testing. There's a flat, residential side street nearby, but it's only a half mile long, and the locals might be unnerved by a strange car being driven very slowly, back and forth, and back and forth. Poor neighborhoods have poorly paved roads, rich neighborhoods have nervous retirees and housewives.
I could drive over to the university in the evening and pretty much have the place to myself, and people would assume I'm just an engineering student working on a project.
One interesting road near me is actually a three-lane urban
expressway. That could be useful for coasting down from 60 mph to 50mph, and turning around for another run is easy. I'd rather have a country road, but there aren't any within fifteen miles.