When an air filter becomes clogged or blackened by use over time, it restricts the amount of air that flows into the engine. This causes the engine work harder and use more gasoline than it should in order to run efficiently. Replacing the air filter regularly and before it becomes overly clogged will return the air flow back to normal levels which will increase gas mileage. You could say that a clean, regularly replaced air filter will allow your car to get the gas mileage it was supposed to have.
A high flow air filter is one that will increase the amount of air getting into the system. By increasing the amount of air, it stands to reason that the amount of gas required to run the vehicle is reduced which means you will get more gas to the gallon which will save you money every time you drive your car.
Have your air filter checked regularly to see if it needs replacing. The cost of replacing the filter is low and you will not only increase your gas mileage, you will reduce the wear and tear on your engine because it will not have to work so hard.