Re: Need Help - Magnaflow mufler and K&N Cold air intake upgrade issue with mpg
What happens when you add these things is that it lets your engine breathe easier, allowing more horsepower to the wheels than trying to take in more air. But if you step on the pedal a lot, the engine adds more gas than stock to correct the air/gas mixture. If your computer decided that it need more gas instead of working less to correct the mixture, it would suck more gas and lower your milage. If you stepped on it the first few times after you cleared your computer, this could have possibly happened.