I dont think there is an answer. Its like a trick question considering the image above of how the tranny works to get the power from the things that spin to the things made with rubber rings.
THe prius has its unique aero shape, then other modifications to boost mpg such as the coolant recovery system, the atkins cycle engine as mentioned and I am sure a hot of other features that should be tested with the engine and body if you could in theory just flip a switch like in the IMA system of an insight.
Id venture to say with my experience with the 2nd gen Insight around 35mpg.
FYI, the inclusion of a phev kit nets 10-15 mpg without modifying your driving techniques. Driving to take the advantage of both a hybrid and phev kit? You can easily exceed 100 mpg.
