In recent Renault-Nissan advertising campaign, car maker is persuading us, that its newest green cars, equipped with eco2 dCi 1.5 liter engine (new Clio, new Laguna) produce just little more CO2 exhaust than humans when running… As Renault explained to us, they used study "Tissue Engineering, Palsson et al., 2003"of USA agriculture ministry,  where authors claim that in every liter of exhaled air there are approximately 2 grams of CO2. An average human exhales 5-10 liters of air in minute (so 20 grams of CO2). When running, breathing is accelerated to 35-40 breaths per minute, with excessive strains, even to 60-70 breaths per minute, which leads to CO2 exhaust of around 15 - 20 grams more than when breathing normally. When calculating in the time required to to run for 1 kilometer, the total sum of CO2 exhaust from human (and score is preety conservative) is close to that of a green eco2 diesel car from Renault… Let  us just describe that numbers representing mileage in Europe are defined with string of three numbers, like: 5,2/4,1/4,5, meaning how many liters car uses to drive for 100 kilometers, and first number means city, second open road and third and last is combined cycle). For converting between mpg and litres/km, use this form here