Although not complete, this is pretty interesting stuff particularily when it comes to smog checks. The impact is pretty low among hot rodders on the environment, especially in Vermont...
http://www.sema.org/main/semaorghome.aspx?id=56863
It is not a big surprise that Cal has the most and highest percentage at almost 6% of the total vehicles in the state, there is no winter. It is interesting that they don't feel that a number that large is an impact to air quality. It is useful in the fight in other states...