Regarding - why does increasing Cr (or rV) not increase overall fuel economy?
Remeber there are two efficiencies here, which are COMPLTELY different, thermal efficiency (formula that uses Compression ratio) and mechanical efficiency.
For a better THERMAL efficiency, high CR is always better. But it is just a theoretical number. Mechanical efficiencies are based not on compression ratio directly, but more about material properties and engine speeds and loads and VE.
As an engine goes faster, mechanical efficiency drops, as does VE (cylinder filling). Lower compresssion can increase mechanical efficiency due to less forces (friction > heat) onto contact surfaces.
Think of it as two lines on a graph, compression ratio x axis, efficiency y axis. as comprssion increases, mech eff goes down. So top left to bottom right. Thermal eff goes up, so botom left to top right. We have a big X. Very simplified, i know but it is just an illustration.
Middle of the X is the best point for efficiency. Seems to be 17:1 according to many sources on a diesel, but would expect 18:1 would be as low as you can go on these engines...
What was the numbers for the increase in compression volume too get 18:1 again? I think I have an idea...