98 Honda Civic Terrible Mileage?

I have a 1998 Honda Civic Coupe. Short Ram Air Intake and it has this knob that you can change how much air goes in, it's on high.
I also have a Skunk2 Intake Manifold, Skunk2 Polished Headers, Skunk2 Whole Exaust system.
Brand new tires, lightweight rims.
I'm getting around 200 miles on a tank and it can hold 11.9 gallons which equals to 16.8 MPG which sounds Way off since Google says civics should get around 34 city and 44 hwy
Any ideas? My check engine light is on and it says P0420 which is o2 sensor

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