The carbon emitted by pigs is the one that already belong to the carbon cycle, depending on the kind of carbon (CO2 or methane) may stay more or less in the sys

The carbon emitted by pigs is the one that already belong to the carbon cycle, depending on the kind of carbon (CO2 or methane) may stay more or less in the system but there is nothing new there. The cars (or if you want, their fuel), on the other side, brings new carbon to the table.