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You're ignoring one other factor: Load. Engines tend to be more efficient at making power when under larger loads, generally around 70-80% or so.
Check the BSFC charts, they have the info you need. The "islands" on them are the amount of fuel used per unit power made, while the X and Y axes correspond to RPM and load.
Note that when you are in a steady-state cruise, your load will be quite small, and will depend on quite a few things (rolling resistance, aero drag, temperature, yadda yadda yadda). So the BSFC charts (and the fuel/power vs. RPM charts) are much less useful at that point... But for acceleration, they're kinda nice to see!
-soD
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