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I am no pro. I came here looking for answers but here is what I know:
On boats even well matched diesels fitted with right props often get decent efficiency on certain narrow range where most of the HP for given RPM is truly needed.
With variable pitch propeller (essentially a tranny equivalent for cars) one can drastically improve the fuel burn.
(rough explanation)
What one does is set rpm with throttle, then adjust the prop (essentially upshift) until the engine is near struggling to keep the rpm. I think you can get a pretty good adjustment just by ear but often pyrometer / exhaust temp gauge is used to gauge if the engine is working at full load for the given RPM.
So this does confirm the original posters findings - a diesel engine does waste HP at given RPM and the only way to maximise thsi would be to always load the engine. Naturally because of engine design certain rpms can be more efficient than others but based on solely that fact "high gear" would always be the best.
Now where is the balance of straining the engine if lugging with low rpms etc. I do not know. That is why I came here - to confirm if it indeed is correct to always try to drive at the highest possible gear (reasonable rpm).
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