[fatuna]

Fair question! Carbon had an atomic weight of ~12, oxygen has an atomic weight of ~16. So carbon is responsible for 12/44 part of the weight of CO2. If we assume plastic to be 80% carbon, then 1kg of plastic would produce (0,8*44)/12~= 2,9 kg of plastic. So probably plastic is less than 80% plastic :). But then number makes sense, similarly for diesel.