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The plain BSFC charts are great for showing you what RPMs you use to accelerate, when the aim is to add energy to the car (in the form of velocity) in as efficient a manner as possible. I believe that the peak BSFC island for gasoline-powered cars is at a lower RPM than the peak torque is, just from my experiment with accelerating to 2000 RPM versus accelerating to 2500 RPM.
Cruise is a different matter. You would need to figure out how much power is required to run at a given cruise speed (and assume a flat road, no wind, etc.), then turn that into torque at a given RPM. Then you could plot that on the chart, and repeat for different speeds. That may give you an idea of what speed is most efficient at cruise for that given car/engine/gear combination.
If you want to change the overall gearing, then you need to re-figure the torque/RPM curves for each change of gearing. And, of course, the moment you encounter any wind, or go up hill or down hill, the whole "power required to maintain a given speed" has to be re-figured...
All that said, I still think that the most efficient cruising RPM for any gasoline-powered car out there is "as low as you are comfortable with".
Diesels have different torque characteristics, and different BSFC maps, but I wouldn't be surprised if their results were similar--that peak efficiency is almost always at lower RPMs than peak torque.
-soD
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