Conversion of 21 MPG (USA) in L 100km (liters/litres per 100 km) is 11.2 liters per 100km. Cars that get 21 mpg in USA would consume 11.2 liters of petrol if it was driven for 100 km in Europe.
If in the UK and you are asking how many litres per 100 km would be consumed for a 21 mpg car (UK) in imperial measure, then we would convert 10 UK mpg as 13.5 litres per 100km. This means that a 21 mpg car in the UK would consume 13.5 litres of petrol if driven for 100 km.
Just to clarify:
21 MPG (USA) = 11.2 L in 100km
21 MPG (UK) = 13.5 L in 100km
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