IDK
My last trk -Freightliner with Series 60 would often get 9 MPG
It is hard to have a control factor in trks, as weight and loads are so variable.
Trks with an enclosed trailer called "Vans" have a constant, called Aerodynamics.
The largest problem is at the rear, believe it or not, it needs to be tear drop shaped .
Over the last 30 years, we have seen plastic bubble doors added to the rears of some to lessen the drag.
For instance, going down over a pass like Snoqualmie Pass with a 50 foot power boat I did not even need to use the jakes as there was such a parachute effect.
Whereas a 50-foot sailboat that is pointed at the front and almost pointed at the rear was like not having a load at all, except for the weight.
Lots of braking is needed!
As said the control factor is a hard one in the real world
