To add to a thread here some time ago, I just refilled mine from empty earlier and got 542 miles out of the tank. From what I then had to put in (84L), that worked out at about 29mpg. In theory I had 9L left so I could have seen nigh on 600 miles if I'd run it dry.
Mixed driving from motorway to city. Not heavily laden. Quite heavy footed. Van Aaken Smartbox fitted.
So, whoever was getting less than 350 out of a tank, you may want to check it out. Checked for diesel on the driveway?
David