I can regularly get 40-41mpg if I want - on my way to work and back.
22 mile run, on a mix of M11/A406/A13
I always fill up my tank when it gets to the last quarter mark, and get around 320-330 miles to that point - with the computer still showing another 80-100 miles to go.
On a longer run, I reckon I could get 420-450 miles to a tank if I wanted.
In fact Im driving around 300 miles up North on Tuesday, then back later in the week. Wont be putting my foot down since I will have the wife and kids with me. Will see what I get and report back