It's a diesel engine in a
It's a diesel engine in a tiny B-segment, so I don't doubt the figure at all. The trucking industry has known the benefits of long-pipe EGR (aka cold-loop EGR) for years and variable-geometry turbos are a huge economy improver as well.
Both VW and Audi use a traditional EGR for their TDIs, which adds choke to the engine (sending hot soot into the chamber). They make up for this with higher turbo pressure, but that can only go so far. I think this new way of doing things will soon become the norm.
Besides, if a TDI Jetta Wagon (with a much larger bore) that weighs almost 40% more than the little Megane can get 48mpg, I see little reason to doubt Renault's numbers.
I can't link to the original media release from Renault here, but it states "3.5 litres/100km" NEDC, which I translated to be 67mpg US, though the math actually says 67.2 (I originally did it using a conversion chart).
You should be able to hit "edit", Frank, and see the link to sources inside the system.