EPA paper on medium duty and heavy duty trucks.
 

https://acquia-dev-nhtsa.dot.gov/sit...chstudy-v2.pdf

Lots of amazing stuff in here including how a 2011 6.7 Ram cummins runs the EPA cycles with 50% payload added (page 106) then they go on to adjust many variables.

I love the graph that shows that a 50% decrease in fuel consumption leads to a 100% increase in fuel economy. Who would have thought?

... but from page 67 onwards it gets very noteworthy, describing the measured effect of several fuel saving technologies.