Another random story of mine...
My dad built a 4x4 truck out of a OLD car front end (early 70s I think), v8 350 engine (from an olds cutlass, early 70s again), box custom made and very heavy ~1 ton alone, with large tires, duals in the back, 2nd 3x sized battery for backup/jump starts and he managed close to 20mpg with that thing. This was back when I was like 8, so don't remember a lot about it, but I do remember him saying it "idled" down the road, something like 1100rpm @ 55mph. I'm thinking at that rpm, the engine was loaded close to 80% to be efficient? I know for a fact the areo was not good besides the fact the cab was a car. If I remember correctly, the truck weighed in at around 6800lbs empty at the scrap yard.
Kind of an interesting read, explains a lot of the "storys" about the early 70s olds 350 engines in my family (used as truck engines a lot, good tourqe and last well 300k miles on some). Scroll down 1/2 way in the 350 section.
Oldsmobile V8 engine - Wikipedia, the free encyclopedia