Why diesels are most efficient around 1,800 rpm

That reminds me. Emissions are something else to take into consideration on stock trucks. Load at low rpms create high cylinder temps(and EGTs) which create Nox. The EGR will open to introduce cooled exhaust gas in order to lower those temps to reduce Nox which reduces efficiency. Something that doesn't happen at higher rpms under the same amount of load. I don't believe these BSFC maps take emissions devices into consideration in these numbers.