Note: This discussion is Only focused upon Fuel economy, not speed/torque/performance/horsepower/throttle etc.

WAI: Warm Air Intake
CAI: Cold Air Intake

A good installed aftermarket CAI increases performance/hp/throttle etc,
but fuel economy is only gained by a well-designed & installed WAI,
otherwise cars would have come with a built-in factory assembled CAI to save mpg's.

" A warm air intake or WAI is a system to decrease the amount of the air going into a car for the purpose of increasing the fuel economy of the internal-combustion engine.
-- Source: Wikipedia

  • " The hot air intake functions by drawing in air from near the engine, which has been heated due to internal combustion. This warm air causes the fuel mixture in the car to drop, thereby lessening how much fuel is used and increasing mileage. "
    -- Source: Ehow

Need more experts & veterans on this topic for their experience & explanation, respectively.
Thank you.