[Big Dave]

The commercially available diesel engines are pretty well sorted out from an economy standpoint. I, too have tried and gotten only trivial improvements.

Diesel vehicles can get better MPG with gearing changes. Diesel vehicle have to use the same gearing used for gas engines because of economics of scale.

Erni'e TDI Beetle uses a 3.89:1 ratio, but the TDI is very torquey and could easily push something in the range of 2.93 or 3.08 and such ratios will greatly improve MPG, particularly in conjunction with aerodynamic improvements.

If you think you can improve MPG with engine mods, you are kidding yourself.