The formula with 45 substituted
km/L = 45 × 0.425144 = 19.13
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 45 mpg (US) is 54.04 mpg on the imperial gallon, 5.23 L/100km and 19.13 km/L. If your 45 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 15.93 km/L instead.
What 45 mpg (US) means in practice
45 mpg (US) is 19.13 km/L, hybrid territory in the US and diesel territory in India. It is a little under the 20 km/L figure that counts as excellent almost everywhere.