Unless you have modifications up the ying yang, the stock air filter will give you all the mileage and HP the engine has to give.
You cannot increase mileage with a low restriction air filter in a closed loop system which uses a MAF (mass air flow) sensor to control the air/flue ratio. More air means more fuel will be used as the ecm is fixed to allow 14.7 lbs air to 1 lb of fuel, so no matter how much air you can get in, the ecm will compensate the fuel to match it.
I know many claim increased MPG but that is no proof of anything. Its all marketing hype to get you to buy their products.
