As a side note from a 'motor head' I suspect a more fuel-efficient diesel is possible here - but at the expense of using something that isn't available at every truck repair place. I'm thinking not so much compression ratio (Riccardo's classic work) but decoupling the stroke from crankshaft diameter a la marine medium speed diesels. A longer stroke extracts more energy at the expense of a taller engine, and as this does not have to be coupled to a transmission or drive line, there is much more flexibility in engine placement.