Convert kilometer per litre to gram of CO2 per km (Diesel)

Continental Europe states fuel consumption in litres per 100 km, where lower is better; the UK and US use miles per gallon, where higher is better, and Japan, India and much of Latin America use kilometres per litre. The two kinds of figure are reciprocals: 235.2 ÷ mpg (US) gives l/100 km. Mind the gallon: the imperial gallon is 4.546 l, the US gallon 3.785 l, so the same car scores 20 % more mpg in Britain than in America. 6 l/100 km is 39 mpg (US), 47 mpg (imperial) and 16.7 km/l.

The CO₂ figure follows directly from the fuel burnt, because almost all the carbon in the fuel leaves the exhaust as CO₂: a litre of diesel makes about 2.65 kg, a litre of petrol about 2.32 kg and a litre of LPG about 1.9 kg. So 5 l/100 km of petrol is 116 g/km and the same of diesel 133 g/km; diesel's higher carbon content per litre is offset in practice by lower consumption. The values are approximate, as the exact figure depends on the fuel blend, and official ratings come from the WLTP test cycle rather than from road use.