"At the end of our week with the Prius c, we were seeing 5.6 L/100 km"
At you drive you up hills all day long ?
The only time I have mileage in our 5 year old Prius was last winter , wife would let it run to warm up and a trip to Halifax , the car was loaded with 4 people and luggage to match driving at 130 km/ hr
Most times we are in the 4.5-4.8 range