Gary, I consider the Prius nothing more or less than a tool. A vehicle used to get me from point A to point B. Sure, I could likely experiment, as does wayne, and get improved FE, or maybe even Stellar FE, but to what end?

Drive, she said, and drive, he did.

Personally if I really wanted to improve FE I would be adding an inverter style genset of the approximate capacity (5KW??) required to move the vehicle along briskly on a flat roadbed. The genset would idle along or maybe not even be running unless the hybrid battery dropped below, say, 1/3 charge, and then the genset would run WOT, the most efficient region for any otto engine, until the batteries were again topped off.

There would then be no reason EVER to use the OEM ICE for hybrid battery charging, for which it is EXTREMELY inefficient. Only regenerative braking and/or the more efficient (vs the ICE) genset would provide enough energy to keep the hybrid battery in an acceptable state of charge.

Additionally the genset could be run from my natural gas supply at home with a commercially available CNG fueling "station" currently available in the marketplace.