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It's all about PPM (parts per million). They release less emissions as they drive however the emissions that they do emit have higher levels of not-allowed stuff.
So we allow big diesel trucks because they have lower parts per million of the naughty bits but they in the end pollute more.
The EPA reaaally needs to adjust how they measure emissions. I think a PPM/mile or something method would be much better off.
at least that's my understanding. I could be wrong.
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