Conversion of 39 MPG (USA) in L 100km (liters/litres per 100 km) is 6 liters per 100km. Cars that get 39 mpg in USA would consume 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 39 mpg car (UK) in imperial measure, then we would convert 10 UK mpg as 7.2 litres per 100km. This means that a 39 mpg car in the UK would consume 7.2 litres of petrol if driven for 100 km.
Just to clarify:
39 MPG (USA) = 6 L in 100km
39 MPG (UK) = 7.2 L in 100km
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