Conversion of 51 MPG (USA) in L 100km (liters/litres per 100 km) is 4.6 liters per 100km. Cars that get 51 mpg in USA would consume 4.6 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 51 mpg car (UK) in imperial measure, then we would convert 10 UK mpg as 5.5 litres per 100km. This means that a 51 mpg car in the UK would consume 5.5 litres of petrol if driven for 100 km.
Just to clarify:
51 MPG (USA) = 4.6 L in 100km
51 MPG (UK) = 5.5 L in 100km
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