This New Scientist Article suggests that an inactive person exhales about 350 litres of carbon dioxide (CO2) per day.
Several online sources quote a US Department of Agriculture (USDA) study that suggests an average person's respiration generates approximately 900 grams weight (450 liters volume) of CO2 per day. These sources reference a publication "Your Role in the Greenhouse Effect" by Jerry Hannan. I cannot find this original source online.
As noted at SEED FAQ 1171, the CO2 released by burning gasoline (petrol) is approximately:
- 10.7 Kg per imperial gallon - about 5.4 m3 (5,400 liters)
- 8.9 Kg per US gallon - about 4.5 m3 (4,500 liters, 159 ft3)
- 2.4 Kg per liter - 1.2 m3 (1,200 liters)
So - based on the USDA estimate, burning one US gallon of gasoline produces ten times more CO2 than one person produces naturally in a day.