In 2011 there were 181 super clean vehicles registered , 928 in 2012 , 1,546 in 2013 .
2,554 were registered during the first seven months of 2014 .
In 2011 there were 181 super clean vehicles registered , 928 in 2012 , 1,546 in 2013 and 2,554 were registered during the first seven months of 2014 .