in my opinion there is no such car that can be the best in mileage ,, fuel efficiency depends on driving style if you drive calmly around 2-2.5k rpm you will probably get a mileage of 12.5-16 km/l iam talking about any 1300cc-1800cc etc may be those 6600cc-1000cc would give you better mileage than 1.3-1.8 liter
google for hybrid vehicle,,,,, these are cars that runs on two distinct powers like an petrol engine power unit and battery driven electric motor....