I have been searching and have found a lot of conflicting opinions so perhaps someone in the know can straighten me out.

If both a 300-turbo diesel and a 240d were driven to maximize mpg and both were well maintained what mpg would get?
I assume that the 240d would get better mpg since it is a 4 cylinder instead of a 5.
And while the turbo on the 300d increases performance I assume that it either does not add any efficiency or perhaps detracts from it.