Do KN air filters work?

On a daily driver they are of little or no benefit, and are not cost effective. They provide no noticeable increase in performance or mpg. In racing applications they do improve performance. They increase air flow in the very upper rpm ranges.

It also depends on what type of K&N air filter you get, if you spend the money on a cold air intake you will get better MPG on the highway and you will get more horsepower. If you get one of their stock replacement air filters it won't be as noticeable but you will be able to keep that for the rest of the cars life and instead of replacing it you can just hose it off.

K&N filters use oil to trap particles, which relative to the size of dust particles is similar to glues. They are also reusable by cleaning them with a hose, allowing them to dry, then reapplying the filter oil. The above posters are correct in that the filters make a negligible difference with typical driving; the change in drag is the square of the change in airflow speed.

And they make you burn through more fuel, especially if you have a stock CPU and it feeds more air through the intake system the CPU needs to balance the fuel to air ratio hence using more fuel to catch the increase of air introduced into the system.