Here is a prefect article showing power ratings and fuel consumption rates of various diesel engines with different characteristics. . Notice what happens to the fuel lb/hp-hr after 1,800 rpm for most of them. It starts to drop off significantly and most cases the lower rpm where the engines first reach peak torque is not the most efficient in regards to hp-hr.
It also points out why engine manufacturers make different power ratings for the same engine and the different of short intermitted power output versus sustained(continues) power output. Having more power than needed will reduce fuel economy and reduce engine life (depending on how far you go past what the engine components can support) while too low of power for the application will reduce life due to being at higher loads more often. The higher power units will not require as high rpm to do the same work, but will see lower engine life if that added power is always utilized at higher rpms or too low of rpms.
Making Sense of Diesel Engine Specs
