I'll add something, based on the map above, if we set 400 lb-ft, or roughly 40% torque put, as the "lightly loaded" driving situation. i.e. cruising in city traffic. Then it's definitely better (strictly in the mpg sense) to "lug" the engine between 1000 - 1400 rpm.
If we let the engine creep up to even 2000 rpm, fuel efficiency can drop well into the yellow / red zone.
But OEM probably factors in many other parameters, DPF life can be one of them, or "passing power" / "driver satisfaction". Hence they'll program shift points higher than 1400 rpm.
Funny how times change, we now call 400 lb-ft light load, whereas for the first ISB that was foot to the floor...
