Perhaps doing a gearbox reset may help, someone here will be along to tell us how it's done, then it learns to adapt to your driving style. I've got the 2015 model, AMG line, with the 7g box, and like you I was told I'd get over 50mpg, after a few trips I was struggling to get more than 40, so it went back in to the dealer, and I beleive their check is to drive the car at a steady 56 mph, reset the counter and see what the guage then says. They came back and told me they were getting the right sort of mpg level, and in the 2 years I've had it I've averaged 42mpg, including some very long motorway trips to southern Spain with the cruise control on 65mph. I very occasionall see over 50 on a mototrway run, but mostly around 46 to 48, again doing 70 on the cruise control. Now when I see over 42 mpg, I think I'm doing well. And I drive like an old f@rt!