Efficiency labels answer the same question from opposite directions. L/100 km states fuel used for a fixed distance; MPG states distance travelled on a fixed fuel volume. The number that looks larger is therefore not automatically more efficient.
Read the direction of the unit
With L/100 km, a lower number means less fuel for the same 100 km. With km/L or MPG, a higher number means more distance from the same volume. Comparing raw values across these systems is misleading; convert them to one common unit before evaluating vehicles.
US and UK gallons are different
A US gallon is 3.785411784 litres; a UK imperial gallon is 4.54609 litres. The same vehicle therefore has a numerically higher UK MPG rating. A figure of 40 MPG is incomplete unless its standard is named, especially when a vehicle listing crosses markets.
Use the correct reciprocal conversion
Convert US MPG to L/100 km with 235.214583 divided by MPG; use 282.480936 for UK MPG. Because the relationship is reciprocal, doubling MPG halves L/100 km rather than subtracting a fixed amount. Keep several decimal places during conversion and round only the displayed result.