2013 Volvo Vnl d13 motor, fuel economy 5.5-5.8mpg no engine code.

Hey guys, I need help. My truck is 2013 Volvo Vnl 780 with D13 motor that running like crap getting around 5.5-5.8 mpg, my brother got the same truck and getting 6.7-7.4mpg. Idle normal 0.7-0.8 gph. No engines code. It’s not deleted or anything.

side note (under load it’s feel slageash at 1350rpm or below. Before it would do around 1000-1100rpm)