The formula with 35 substituted
km/L = 35 × 0.425144 = 14.88
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The MPG to km/L Converter explains where each comes from and gives a longer table.
The same 35 mpg (US) is 42.03 mpg on the imperial gallon, 6.72 L/100km and 14.88 km/L. If your 35 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 12.39 km/L instead.
What 35 mpg (US) means in practice
35 mpg (US) is 14.88 km/L, close to the 15 km/L that marks a good petrol car in km/L markets. A Japanese compact advertised at 14.9 km/L on WLTC is, in American terms, a 35 mpg car.