The formula with 38 substituted
L/100km = 235.214583 ÷ 38 = 6.19
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 38 mpg (US) is 45.64 mpg on the imperial gallon, 6.19 L/100km and 16.16 km/L. If your 38 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 7.43 L/100km instead.
What 38 mpg (US) means in practice
38 mpg, or 6.19 L/100km, is about the ceiling for non-hybrid petrol cars in the US, achieved by a few small hatchbacks with continuously variable transmissions. Anything above it almost always has a battery.