The formula with 23 substituted
km/L = 23 × 0.425144 = 9.78
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 km/L Converter explains where each comes from and gives a longer table.
The same 23 mpg (US) is 27.62 mpg on the imperial gallon, 10.23 L/100km and 9.78 km/L. If your 23 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 8.14 km/L instead.
What 23 mpg (US) means in practice
23 mpg to km/L is one of the most-searched conversions in this family, probably because 23 mpg is a common SUV rating in American reviews read abroad. The answer is 9.78 km/L, a figure Indian buyers would call poor for a hatchback and normal for a large SUV.