I just looked up some data on the Gardner 8LXB. It's rated 175hp at 1500 RPM. At full power, it burns 8.4 gal/hr, giving an output of just over 20.8 hp-hr per gallon. That efficiency seems to scale very well across the whole RPM range for that engine.
FWIW, the most efficient modern small-ish diesels are in the 20 - 21 hp-hr / gallon range. Some may get as high as 22. Many of the smaller, naturally aspirated engines are down in the 18 - 19 hp-hr per gallon range. 2 stroke Detroits are typically around 16 - 18.
So the Gardner is still quite efficient by modern standards. Not meaningfully better than most modern engines, but it's just as efficient as the majority of them.
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