You really need to specify whether that 50 miles per gallon is in US or UK gallons ...
US gallon is 3.78 Liters, UK gallon is 4.54 Liters.
US gallons: 50 MPG = 21.26 km/liter UK gallons: 50 MPG = 17.70 km/liter
50 MPG (UK) is 41.6 MPG (US).
In my Hyundai Accent (Korean) I get 34 - 36 MPG (US) depending on how I drive it. It is not nearly as efficient as a Renault Twingo or my grandma's Toyota Aigo (awesome little car).