[RedDevil]

The thing that strikes me in the graph is the curves that follow a set amount of power.
While the area around 2500 revs is most efficient when you consider equal torque, it definitely is not when you look at equal power.

At lower revs you need more torque to get the same power and that dramatically shifts the balance towards lower revs.
The 7.5 kW line has an optimal BSFC (around 250) just below 1000 RPM.
The 15 kW line hits 235 around 1300 RPM.
The 30 kW line hits about 225 just above 2000 RPM.

That seems to suggest that when maintaining speed on the level (10-25 kW at moderate highway speeds) you should select as high a gear as possible and only shift down when you go over 80% throttle.

Maybe it is as simple as this:
If you go over 80% throttle shift down. If you go below 60% throttle shift up.