The formula with 28 substituted
km/L = 28 × 0.425144 = 11.90
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 km/L Converter explains where each comes from and gives a longer table.
The same 28 mpg (US) is 33.63 mpg on the imperial gallon, 8.40 L/100km and 11.90 km/L. If your 28 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 9.91 km/L instead.
What 28 mpg (US) means in practice
28 mpg (US) is 11.90 km/L, the sort of figure a compact crossover returns. Indian ARAI ratings for similar vehicles read higher, around 14 to 16 kmpl, because the test cycle is gentler.