CO2 gets 73% of its mass from the oxygen it uses for combustion and not from the carbon in the gas (which is something like 85% of the weight of gas). So take that 133lbs multiply by 0.85 and divide by 0.27 and you get 356lbs or 71lbs per 100 miles as a rough back of the envelope calculation. Pretty close to the 30kg (66lbs) listed by the parent.