I would love to see the manufacturer taxed on the difference between advertised and real-world measured pollution (perhaps exponentially, if they emit more pollution than they advertise -- this would greatly incentivize conservative accuracy in fuel economy and emissions specs).

But no matter what scheme we propose, I do worry that this would become an adversarial "gaming" situation a bit like hackers vs DRM -- no matter what the rules/criteria to reduce dangerous emissions, manufacturers will employ people specifically to circumvent them in any and all ways possible. This isn't to say we should stop trying to curb emissions -- just that they must be ongoing and open to change and adaptation to new "attack vectors".