The best you can get from this car is around 17-18 km/l (according to Daihatsu Japan).
Here are few Specifications:
DAIHATSU COPEN
Exterior dimensions (LxWxH), mm: 3395 x 1475 x 1245
Wheel base, mm: 2230
Min.turning radius, m: 4.6
Fuel tank capacity, l: 40
ENGINE
Displacement, cc: 659
Max.power (Net), kw(PS)/rpm: 64 ps (47.07 kw) / 6000 rpm
Max.torque(Net), N*m(kg*m)/rpm: 11.2 kg*m (109.83 N*m) / 3200 rpm
Engine type: Water cooling serial 4 cylinder DOHC16 valve turbo
Fuel system: EFI (electronic fuel injection)
Turbocharger: Turbo
Power steering:Yes
Tires size, front:165/50R15 73V
Tires size rear: 165/50R15 73v