The blistering, low-volume M6 editions aside, BMW's 6-Series earns decent fuel-economy ratings, thanks to an efficient turbocharged entry-level engine and eight-speed automatic transmissions.None of the powertrain choices can truly be called green, but the turbo in-line six in the 640i Coupe is rated at 22 mpg city and 33 mpg highway. The EPA pegs the convertible at 20 mpg city and 30 mpg highway, the same as the 640i Gran Coupe. The two-door 640i coupe or convertible, with all-wheel drive, is rated at 20/29 mpg, as is the Gran Coupe.
With the V-8 found in 650i versions, the rear-drive body styles all get 17/25-mpg ratings, while all-wheel-drive versions are rated at 16/24 mpg.
Those M6 editions still are impressive, though only on a relative scale, compared to their V-10 predecessors. Today's M6 Gran Coupe, Convertible, and Coupe models are rated at 14/20 mpg with the automatic, 15/22 mpg with the manual, which is some 23 percent higher than in the previous-generation coupe and convertible (the Gran Coupe being a new addition to the lineup).