What kind of mileage can I expect pulling a gooseneck trailer with my Dodge 3500 now that I have the Diablo installed? I was getting 14.5 before the addition. I have the program set on economy/tow. Any info would be greatly appreciated. Thank You!

I have had my Diablo installed and loaded in my Dodge 3500, Cummins Diesel. How accurate are the readings on the factory overhead readouts panel? Or are they accurate after Diablo is loaded? My reading says 39 miles per gallon average. Before Diablo the readout was 22 mpg. HELP Please!