I currently have a 520D and for a full tank approx. £70 I can get a months worth of driving out of it. Which is work commute (A Roads) and occasional Motorways. This last month included a 350 mile round trip. I'm happy to toot along, but want that get up and go if needed.
I'm spending over £15K to get one so I want my monies worth (as we all do). So reliability is also key. I owned a LS400 about 12 years ago and apart from poor traction in the wet it was superb. I have also owned a Toybota Avensis which was a 2 litre petrol automatic. and I struggled to get 300 miles from a full tank.