The formula with 20 substituted
L/100km = 235.214583 ÷ 20 = 11.76
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 20 mpg (US) is 24.02 mpg on the imperial gallon, 11.76 L/100km and 8.50 km/L. If your 20 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 14.12 L/100km instead.
What 20 mpg (US) means in practice
20 mpg is a round-number benchmark that separates trucks and large SUVs from everything else, and one of the most-converted values on the site. At 11.76 L/100km, it is the figure most three-row SUVs and mid-size pickups sit close to.