Considering on this website they say you can figure out the amount of CO2 emissions per km based on the fuel mileage, I'd suggest they are saying the rate of emissions is proportional to the MPG (given their constant).
This makes sense to me because there are so many carbon atoms in a gallon of gas. When burnt (correctly), the CO2 emissions are going to be constant. According to this website:
About 19.64 pounds of carbon dioxide (CO2) are produced from burning a gallon of gasoline that does not contain ethanol. About 22.38 pounds of CO2 are produced by burning a gallon of diesel fuel.
To me, that is directly proportional.
That would be the book answer. It would also indicate the vehicle which is being tested is running perfectly. If, on the other hand, it isn't running perfectly, you could possibly have a much higher rate of carbon monoxide (CO) being emitted, which would mean your CO2 levels would be lower. As long as the vehicle is running correctly or the catalytic converter is keeping up with output, the level of CO2 is constant and therefor can be measured from the MPG of the vehicle.