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Typically, countries that have been more concerned about energy security have chosen to regulate fuel consumption (l/100km) or fuel economy (km/l). Countries/regions more concerned about climate change have regulated greenhouse gas emissions (gCO2e/km or greenhouse gas emissions on CO2 equivalent basis per kilometer of distance traveled). The specific advantage of GHG standards is that they can include emissions of greenhouse gases other than CO2 such as methane, and N2O.
These metrics are largely interchangeable, since we know the carbon intensity of various fuels. Of course, the metric of fuel consumption and greenhouse gas emission have an advantage of being linear. In other words, reducing fuel consumption from 20 l/100km to 15 l/100km saves the same amount of fuel as reducing fuel consumption from 15l/100km to 10 l/100km. On the other hand, as this chart shows, improving fuel economy from 10 km/l to 15 km/l will save more fuel than improving fuel economy from 15 km/l to 20 km/l.