2016 BMW 6 Series Convertible is the newer version with some differences, such as the model year and fuel economy.. First of all, the 2016 6 Series Convertible is more fuel efficient; it gets 1.2 more mpg than the 2013 6 Series Convertible. Among similarities are the body structure, market segment, starting price, horsepower, ground shadow size, height, and maximum occupancy.