I've yet to drive a Prius C, but I have numbers to compare against one on the same "economy run" rally route.
When you drive the Mirage "like a hybrid" (meaning, doing the manual version of Auto Stop & Go, which isn't offered on the North American cars, plus lots of coasting in neutral where appropriate), they're extremely close.
Route: 200 km / 120 miles round trip of challenging, fast twisty country roads, sizeable elevation change (400 m / 1300 ft.), a short bit of freeway driving (~10%), a short bit of sub/urban driving (~10%).
On the identical route in the same conditions...
Prius C: got 4.05 L/100 km = 58.1 mpg US = 24.7 km/L = 69.8 mpg Imp.
Mirage 5-speed, base model: got 3.98 L/100 km = 59.1 mpg US = 25.1 km/L = 71 mpg Imp.
More about the rally in this thread.