45 MPG city, 40 MPG highwayIt looks like the official EPA rating of the Ford C-Max hybrid (a nicely even triple 47 MPG in city/highway/combined) turned out to be over-optimistic by a significant margin. The real-world fuel efficiency of the C-MAX generated some controversy, enough for the EPA to take a closer look. The new numbers are in, and Ford will re-label its 2013 C-MAX with 45 MPG in the city, 40 on the highway, and 43 combined. It's a bit under 10% lower (the percentage is what matters - the number of MPGs can be misleading because 4 MPG is a lot less for a 40+ MPG vehicle than for a vehicle in the 20 MPG range...).
What happened? Well, the EPA allows carmakers to use the "general label rule" to rate vehicles with the same engine, transmission and weight class using the same fuel economy data. So Ford used the Fusion hybrid's numbers for the C-MAX because they share all those characteristics, but obviously the difference was big enough in the real-world to be noticed. The difference in highway numbers, for example, are probably caused by the difference in aerodynamic characteristics between the two vehicles.