Swing emissions

In a recent episode of Mythbusters, Adam and Jamie compared emission levels of three cars against three bikes each from the same periods of the 1980's, 90's, and 00's and found that CO2 emissions generally followed fuel consumption and that bikes generally did better than car counterparts. What was really surprsing was the other types of emissions that were hundreds to thousands of times worse over an identical course. They theorized that the reason was because emission standards are so much more stringent for cars and that a lot more work has been done to clean the exhaust. One of the monitored gasses was nitrous oxide.

Has anyone here seen comparative studies of Silverwing emissions versuses modern cars?