I can't really say. The 587 has a 600 hp ISX in it with an Eaton Ultrashift Plus. The 579 has a 12 liter Cummins and an 18 speed manual. Not really apples to apples there. On the other hand I will be driving a 579 with a 2017 ISX with a 13 speed manual tomorrow for work. I'll see what the dash tells me the fuel mileage is when I get done. I will reset it before I drive.
Alright, 500 hp 1850 lb ft torque. 3.23 rear end gear and a 13 speed. Eight foot trailer gap between the back of the truck cab and front of the trailer. Windy day. Regional haul (mix of interstate and state roads with in town driving) of 550 miles averaged out to be 6 mpg. If I wanted to get better fuel mileage I would have gotten the van trailer closer to the back of the truck. The truck also did not have the full areo package. It did have the top faring but no side fairings.
