Here's a "Fuel Map" of the efficiency of two Prii ICEs measured in "Brake Specific Fuel Consumption", BSFC. http://ecomodder.com...prius_bsfc1.jpg
The chart is for the last two Prii ICEs. They try to only operate on the heavy solid lines. You can see for the 2FR-FXE, the current Prius ICE, the engine operates within a region where the BSFC is a very low 220 grams/ Kw-hr from about 1100 to over 3000 rpm. The Ford graphs would have similar shapes but have different values. If you watch your RPM while driving, it will usually be within this range. The lowest BSFC for the 2FR-FXE would probably be about 2100 RPM from the chart and be somewhat less than 220. That's where you would operated if you used it solely as a constant speed generator. As the above poster pointed out Chevy has found that's inefficient and the latest Volt has even more torque directed to the wheels by gears. By variable valve timing and other computer controlled ICE operation manipulation the lowest BSFC areas are over a fairly broad RPM range. This is the heart of all Atkinson ICE eCVT hybrid designs. The Eco Boosts do not have these low BSFCs. They are designed to produce a lot of power per weight and displacement.
So in order to theoretically maximize your fuel efficiency in that 2FR, the way you would drive would be to run the ICE at 2000rpm producing about 100Nm of torque in the highest ratio that the CVT will allow. Do this until the battery is full, then take your foot off the gas pedal completely until the battery is so low that the ICE comes on again, then repeat.