So reading between the lines here. If I put my right foot on a diet and my manual calculations still yield 5.5mpg I may have a mechanical issue? Over on the trucker's forums they talk about air leaks causing economy problems, I guess it would bark up that tree at that point. Thanks for the insights.
2006 Renegade/Freightliner Coronado 43', Detroit, twin screw, 12spd
2001 Ford Excursion PSD
2017 Ford F450