See the above advert which claims that Renault Megane can cover 1700km with one tank.
According to this link the Megane model has a fuel tank capacity of 60 Liters.
Can the Megane cover 28.3 Kms per liter of petrol?
According to Renault's figures, a Scenic with the dCI 110 engine can do up to 80.7 mpg or 34.3 Km/litre. This would allow a maximum theoretical limit of 34.3Km/litre × 60 litre/tank = 2058 Km/tank.
Here's how the figures are calculated:
HOW ARE FUEL CONSUMPTION FIGURES CALCULATED?
Official Fuel consumption figures are produced for comparison purpose and are measured according to the European Directive 80/1268/EEC. These figures are specific for a vehicle type and not for a single car. Real world fuel (actual) consumption can vary as the fuel efficiency of a vehicle is influenced by driving behaviour, weather and road conditions. The use of technical equipment for example air conditioning, can also effect the fuel consumption. For further information, please visit the Vehicle Certification Agency web site www.vca.gov.uk.
So it's a standard measurement, however it should be used fo comparing car models and not to predict usage of a specific car.
The ad seems to be misleading on that point.