[AeroModder]

I get better fuel mileage with a Fram paper filter, but the K&N cotton filter gives a noticeable improvement in power and response.

There was a study done about the effects of dirty air filters (can't remember the source, but it was posted here somewhere), and the conclusions were on fuel injected vehicles, lower airflow=less gas used=better FE, but lowered performance. On carburated cars, the lowered airflow altered the air/fuel ration and degrated both performace and economy.