The formula with 17 substituted
L/100km = 235.214583 ÷ 17 = 13.84
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 17 mpg (US) is 20.42 mpg on the imperial gallon, 13.84 L/100km and 7.23 km/L. If your 17 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 16.62 L/100km instead.
What 17 mpg (US) means in practice
17 mpg is a typical combined rating for a full-size SUV with a large petrol engine, or a half-ton pickup with a V8. The metric figure, 13.84 L/100km, is the kind of number that makes European readers wince.