The formula with 30 substituted
L/100km = 235.214583 ÷ 30 = 7.84
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 L/100km 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 9.42 L/100km instead.
What 30 mpg (US) means in practice
30 mpg (US) is the classic benchmark for a sensible car, and it converts to 7.84 L/100km. Front-wheel-drive compact SUVs and larger sedans sit here; a British reader would call the same car a 36 mpg car.