Lets not forget there are gasoline engines without traditional throttle - no pumping losses. Diesel fuel has higher energy content per volume, plus they run higher CR.
Anyway, every engine has this https://x-engineer.org/automotive-engineering/internal-combustion-engines/performance/brake-specific-fuel-consumption-bsfc/
Swamps Jeep pulling a trailer has to work outside of best efficiency zone while his neighbor`s pick up is in the sweet spot. That's pretty much it.