A car’s engine uses an air intake in order to retrieve air from outside of the vehicle. The air is then mixed with the fuel, and combustion is the result. This is what propels the car. There is a sensor on the air intake called the mass air flow sensor. This sensor calculates how much air to mix with the fuel.
If you are using a cold air intake, the MAF sensor might not be calculating properly. This means you are not getting the added gas mileage that most cold air intakes advertise. You can bypass the MAF sensor, and allow the O2 sensor to calculate how much air is needed.
Turn off the ignition of your vehicle and open its hood.