The formula with 45 substituted
L/100km = 235.214583 ÷ 45 = 5.23
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 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 6.28 L/100km instead.
What 45 mpg (US) means in practice
45 mpg (US) converts to 5.23 L/100km. Compact hybrids with all-wheel drive and larger hybrid SUVs sit here; it is an excellent figure for anything with a boot big enough for a family holiday.