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UFO, that's a misconception. Remember, the graphs are for brake-specific fuel consumption. That means amount of fuel burned per horsepower generated. If you run the engine at 3000 RPM where it makes 55 Nm of torque, it will add energy as efficiently as possible to the car, but the car will speed up! That's not what you want when you are at cruise.
So the key for steady-state cruising is to find where the engine most efficiently generates the ~15 HP required to push the car down the road at ~60 MPH. And that isn't so easy to see from the graph. And, of course, the amount of power required to keep the car moving at a given speed is different for different speeds.
Interesting that the BSFC peak is around 3000 RPM for this engine, as opposed to around 2000 RPM as we have seen for a number of others.
-soD
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