i just dropped in a k&n air filter into the stock airbox yseterday and took the car for a spin and it seemed like the fuel consumption took a heavy hit. i only did about 15 miles and the guage dropped 2 notches (i know this is not an accurate measure of the petrol but its also not normal compared to how it drops usually).

what is the likelihood that the air filter is causing this? is it possible im just imagining it because im already disenchanted with the car's crappy mpg? could it have been installed wrong? when i put it in a pressed the sides down quite firmly but i dont know if it could be the wrong way or something like that? it looked fine and fit perfectly so i dont know..

 

That's a terrible way to gauge milage. Record your miles for a tank using the trip odometer, and divide that number by how many gallons it takes to fill it back up. That will give you milage. You can do a before and after if you're concerned, but no the K&N (unless you really botched installation somehow) will not give you worse mpg. If anything you should get a slight boost since it gives you less resistance through the airbox. Oh... and if you search you can find all the info you need. This topic has been posted to death several times over.