Clean air is a good concern, but I can't help but correlate how unreliable modern engines have gotten with increasingly stringent fuel economy. I'm all about a cleaner environment, but when any gas savings I see are eaten up and surpassed by repair costs, and overall initial vehicle costs for those technologies, our net expense has actually gone up. I'm not saying some tradeoff shouldn't happen to keep our air clean, but just that the overall picture should be taken into consideration. 

From a financial POV, less reliable high-strung turbo 4s and whatnot seem to cost more in the long run despite saving some fuel. One repair basic negates a lot of costs saving on fuel. From an environmental POV, there's a carbon cost for all the parts, repairs, engine replacements, etc. that need to be factored in for these higher MPG yet lower reliability cars. For example, Toyota going to smaller turbo motors to comply with clean air that are being recalled for engine replacements by the 100's of thousands creates a lot of additional greenhouse gasses. Smelting aluminum for replacement engine blocks and shipped them all around the world creates C02. I don't think it's arguable that an existing, older, low MPG vehicle kept on the road creates less C02 than manufacturing a new higher MPG car.

I was thinking about this recently, and I think a regulation that grants carbon offset credits to manufacturers for building more reliable vehicles should be added to the environmental regulations. As in, instead of just focusing on what comes out of the tail pipe, also consider how much C02 is produced to constantly repair/replace vehicles. Give manufacturers carbon credits if they can produce reliable vehicles that use less resources to keep on the road. It'd be win-win by reducing overall C02 while giving incentive for manufacturing to design and build reliable cars for people.