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Originally Posted by some_other_dave
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.
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Exactly. And this is why Pulse & Glide works. When you are using the engine, it's at its peak efficiency. The rest of the time it's off / idling, and using minimal fuel.
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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.
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That has been my experience as well - lower than peak torque rpm.
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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.
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I rarely / never cruise at steady speed. P&G all the time. My car's most efficient steady cruise speed is somewhere around 35 mph. I hate that it's so crippled with gears that run the rpm so high.
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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".
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Amen to this one!