Hypermiling: Prius G3 versus Prius C

There's more at work than the engine size. The c also has a smaller battery and a less slippery shape.

That being said, it's possible. The engine in the c is the same size as was used in the Gen 2 standard Prius.

 

One of the reasons Toyota upped the engine size to 1.8L was to reduce the engine RPM's at highway speed to improve highway MPG's.

 

What the HSD excels at is utilizing the engine as much as possible in optimal conditions (minimum engine speed with low manifod vacuum/high loading). At higher speeds the power needed can't be met by a smaller engine without speeding the engine up to the point of much less efficiency.
With its lighter vehicle weight the c's engine/traction motor/battery sizing work very well at "city" speeds. At highway speeds it works well but not as efficiently because of the power needed to push it through the air.

 

The C has no belts.
X

 

In general the c has better fuel efficiency off the freeway so if you avoid high speeds then you should beat the Prius easily.