Most of the 2011 BMW 3-Series models are powered by either a 3.0-liter in-line six (328) or a turbocharged six (335). Fuel economy ranges from 18 mpg city, 28 highway for the base 328i sedans with rear-wheel drive down to to 16/25 for the 335ci Convertible. 335i sedans earn an EPA rating of 17/26. On the green scale, that's only average to a bit lower than average for the class.
What gives the 3-Series extra green kudos is the existence of the 2011 BMW 335d in the lineup. That model, featuring a fuel-efficient clean-diesel six-cylinder engine, gets far better fuel economy. With EPA ratings of 23 mpg city, 36 highway, the 2011 335d is more fuel-efficient than a number of sporty four-cylinder models like the Mazda3 or Acura TSX, yet its torquey engine feels nearly as quick as the 335i models.