The formula with 48 substituted
L/100km = 235.214583 ÷ 48 = 4.90
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 48 mpg (US) is 57.65 mpg on the imperial gallon, 4.90 L/100km and 20.41 km/L. If your 48 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 5.89 L/100km instead.
What 48 mpg (US) means in practice
48 mpg, 4.90 L/100km, is the combined rating of several full-size hybrid sedans. It is about what a British diesel hatchback returned under NEDC a decade ago, in a much larger car.