Getting back to the efficiency of diesels. Diesels have traditionally been heavier per horsepower because to achieve their thermal efficiency they have a higher compression ratio and are typically built to be rugged and heavier than the same horsepower in petrol (gasoline) engines. There have been diesel engines in airplanes but they were built differently and from a modern point of view, they are inherently polluters.

And then there was the diesel beetle.

A hiking partner back in the 80s used to tell how he had purchased a Volkswagen diesel beetle. Got about 50 miles per gallon which was better than any motorcycle I ever had. When still new, the head gasket blew and the dealer had to change everything out above the gasket. Next time it blew, they had to change everything on the other side of the gasket!