Originally Posted by ScOoTeR
To immediately address the question of the Taurus vs. the VW and gasoline polluting more per mile driven.
Right now, that is not the case, as that Taurus meets SULEV standards and the VW is only required to meet ULEV numbers. So, while the Ford burns 2.5x the amount of fuel, the VW emits:
5.5x the amount of NMOG (non-methane organic gasses)
2.1x the amount of CO
3.5x the amount of NOx
I agree with your reasoning though - and applaud the thought - but your example ends up being a numbers game for emission standards. Soon, diesels will need to be SULEV emission capable, and you will see a corresponding drop in fuel economy. It will still be higher than most gasoline powered cars, but the gap will decrease enough that you might think twice about buying a diesel.
I lied when I said my thoughts and comments were over.
Now if I am reading what you posted correctly there will be a tipping point where the pollutant level of the VW will be equal or less than that of a Taurus. That point will only come when the fuel savings of a VW will be 5.5x greater than the Taurus.
From my SWAG numbers of MPG earlier and some calculations just now at no point will the VW pollute less since the difference in MPG is less than of 5.5X what the Taurus gets for MPG. I used 5.5 since that was what the highest difference between the two emission standards you posted. The only way for the VW to achieve that tipping point would be if the MPG was at least 110MPG. Correct in my thinking or no.