Geez there are soooo many stories & hoaxes about "wonder devices" that X your power & fuel economy.. you know, the devices that the Arabs immediately pay off the inventors & hide from the public to avoid losing their oil monopoly
Yea a K&N filter will help mileage a bit, by letting the engine breath a bit more freely. Not much of a gain though.. Some of the oil additivies, or using synthetic oil, can help mileage & power becuse they really do reduce rotational friction, so it's easy to see why they work work. Again, only small benefits.
On surprising gain I got was back on my '79 Stang I stuck a Kamei airdam on the front below the bumper.. MAN that thing really did help! Even while driving on the highway you could feel it is causing the frontend to suck down onto the road better, because it was reducing the amount of air passing under the car. Steering response changed so much that it felt like I had completely different tires on the car! And fuel economy got better too. Not sure if modern cars would benefit as much from airdams as the older ones.. new cars are much more aerodynamic.