Been an ASE Master Auto technician for over 25 years and an ASE L1( Engine Performance Specialist) since that distinction became available and since I work mainly with engine performance computers systems and emissions. I've found,like Bill said, most of the mechanical add-ons are junk. Power from airfilters comes from les restrictions, due to bigger holes in the filters for more air( and DIRT) to pass thhrough. I love to see K&N Style airfilters on Diesel trucks as I sell a turbo on most everyone that comes in as the compressor turbine's worn to almost nothing from hitting specks of dirt at 80,000 rpm. Excess oil on the K&N just kills mass airflow sensors, costing upwards ot $600 on a lot of cars. Exhaust systems work, but are expensive. Lucas Oil Products, like fuel additive, work good , too. Bottom line, if a mechanical addon costing under $100 says it gives gas mileage, it probably doesn't, without affecting something else..
RicG