Has to be a 6 pot really, and yes all the 6 cylinders seem to have the same economy, I have an early E46 323i (2.5 engine) and it's average from my daily commute (35 mile round trip mostly B roads) (28mpg) is the same as my friend's M3 but with half the power!

Never had a 4 cylinder one, but an E46 is quite a heavy car so imagine them to be gutless.