The EnergyCS Prius is not a full "gasoline-optional" hybrid. The "assist" range of the larger battery is 50-60 miles, during which range gasoline usage is dramatically reduced but not eliminated completely.

This is because at any speed above 34 miles per hour the gasoline-engine must kick in (for technical reasons, not due to any limitation in battery or motor power output).

In the unlikely event that you needed to travel a long way with low acceleration and at all times below 34 miles per hour, then it should be possible to travel about 40-50 miles before any gasoline is used in EV mode only. (after the battery is depleted the gasoline-engine must switch on, at which time the car reverts to behaving like a normal Prius).

http://mysite.wanadoo-members.co.uk/ecotech/ecot.htm