Air dam / Air foil on cab roof or camper cab over?

You are right, the numbers escape my memory.
The 8V71 in 16 tons Prevost was delivering 6 mpg, at the times when I was holding gas pedal firmly on the floor.
When I was comparing newer diesels 10 years ago, they might be 1 mpg better. Not really good reason to spend additional $ 300,000 on newer rig.
Obviously we are living in times when technology runs on afterburners.
My 3 years old, heavy sedan can deliver 60 mpg at 60 mph.
Even with my heavy foot I am averaging over 40.
ALL THAT WITH NO AIRFOIL ;)