First, read the owners manual. I'll bet there's a spot in there where they say not to use any additives in the fuel.
Second, I will never believe that adding anything to your fuel will boost mpg enough to pay for the additive - snake oil.
Lastly, the 850 lbs-ft of the tq the new diesels make, and the emissions garbage they are saddled with decrease their mpg. The 5.9L Cummins won't be matched for it's mpg and power combination with the power war and new emissions laws the new diesel engines have to deal with. It is what it is. JMHO.