It's hard to say how you can reduce emissions without just throwing all the exhaust into an aluminum bin then using that as fuel, then recycling those emissions.
If you are producing a little more emissions than stock, but are getting more MPG than stock, then you shouldn't really be having a problem. If it really is a concern for you, then buy some new filters and that should be good. New filters can increase HP, MPG, and reduce emissions.
I'm not too much of an environmentalist myself (I mean, the only thing that cars produce is H2O and CO2, which is basically plant food), but I do care about money. The more MPG I get, the thicker my wallet. If that means I save the environment, then I guess I made some hippie friends(?).
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