Why is my gas mileage so terrible?

I have an 07 Ford Focus. In 2010, my dad hit a deer with the car. They fixed everything, but didn t want to spend over $500 on the correct intake for the car and instead bought a sports intake, which causes the car to rev way too high in first and second gear and basically kick forward. I m supposed to be getting 25 MPG in the city, and I get maybe 15-19 MPG, depending. I am definitely not mechanically inclined, so I m not sure what the possible cause could be. Any help?

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