The formula with 27 substituted
L/100km = 235.214583 ÷ 27 = 8.71
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 27 mpg (US) is 32.43 mpg on the imperial gallon, 8.71 L/100km and 11.48 km/L. If your 27 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 10.46 L/100km instead.
What 27 mpg (US) means in practice
27 mpg is close to the real-world average for new vehicles sold in the United States, according to the EPA's Automotive Trends Report, dragged down by the popularity of trucks. It is 8.71 L/100km.