The formula with 23 substituted
L/100km = 235.214583 ÷ 23 = 10.23
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 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 12.28 L/100km instead.
What 23 mpg (US) means in practice
23 mpg is a common rating for mid-size SUVs and older family sedans, and it is one of the values most often converted to metric. It works out to 10.23 L/100km, or 9.78 km/L for a reader in Japan or India.