I know I posted this in another thread, but I'll do it again.
Air Filtration Test
My theories on increased mpg with K&N filters are as follows
1) some EFI systems are in someway fooled by the minor change in airflow.
2) the AFM / MAF is slightly damaged (well documented that the oil on them does harm sensors) and causes a lean condition.
3) (My personal favorite!) most people install them as Cold Air intakes, under the warm hood, typically factory intakes draw from a much colder area (fender/ headlight), therefore what they are installing is a Warm Air Intake.
4) Aliens from Mars
5) (second most probable in my book) It's just a placebo effect. And may be furthered by people maintaining their vehicle when they do the install.