I am fairly sure even if you have the very low geared diff that you have another problem, I have a luton mk6 petrol with a 5.13 diff and i get 20+mpg (4.5 mpl ) ... and its a petrol .
My Jumbo is also a petrol with 5.13 axle and gets over 25 mpg ( 5.5 mpl ) obviously with my luton its not very fast but my Jumbo is and I drive it pretty quick as well I long ago gave up bothering with the diesels but I am guessing the pump is out a bit .