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Consumption in litres per 100 km is fuel divided by distance times 100 — a rate, so lower is better. Kilometres per litre and miles per gallon are efficiencies where higher is better, and one imperial gallon is 4.546 litres while a US gallon is 3.785.
Fuel consumption
L/100km = litres / km x 100; km/L = km / litres; mpg (imperial) = km/L x 2.825
L/100km scales linearly with fuel used, so a 1 L/100km improvement always saves the same fuel. Improvements in mpg are worth progressively less as the number rises.
The imperial gallon is 20% larger, so the same car reads about 20% more mpg in imperial units.