My Scammell Explorer fitted with a 290hp Cummins L10 diesel is doing at least 10 mpg by my rough reckoning, I haven't done a brim to brim test as this would take 148 galls...:sweat:
Due to the low gearing and excess power, only a very light foot is ever needed...
A Diesel conversion does not always have to be expensive, buying a donor vehicle, keeping the bits you need and parting out the rest of it, selling your old engine and any spares you have can keep costs down. If you do the work yourself then you are quids in, after doing mine I was out of pocket around £120 ish..:-D