The formula with 30 substituted
km/L = 30 × 0.425144 = 12.75
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 30 mpg (US) is 36.03 mpg on the imperial gallon, 7.84 L/100km and 12.75 km/L. If your 30 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 10.62 km/L instead.
What 30 mpg (US) means in practice
30 mpg (US) is 12.75 km/L. In Japan that is an ordinary figure for a small non-hybrid car on the WLTC cycle; in the US it is a good one for a compact SUV. Same number, different expectations.