How many of you realize that the vast majority of CO2 emissions is not from cars burning gas, but from coal burning for electricity? Cars do contribute quite a bit, but if we just cleaned up our electricity source, C02 levels would be way down. Too much focus is on cars in my opinion. Below is a study done in 1999 that you may find interesting. The coal numbers are worse today than they were in 1999.
http://www.eia.doe.gov/cneaf/electricity/page/co2_report/co2report.html#electric