The formula with 25 substituted
km/L = 25 × 0.425144 = 10.63
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 25 mpg (US) is 30.02 mpg on the imperial gallon, 9.41 L/100km and 10.63 km/L. If your 25 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 8.85 km/L instead.
What 25 mpg (US) means in practice
25 mpg (US) is 10.63 km/L, a round number in one system that becomes an awkward one in the other. It is the typical efficiency of a mid-size petrol SUV.