There are many theories on this .. constrictive airflow from the piping to the TB presents It's own problems . Certain Air filtraion streams .. Engine Temperatures at start up . Even Coolant Temperaure attribute to Efficiecy of a Engine Load . Factors in Mantaining RPM at a given speed . Engine Detonation Determined by the ECU 's Calibration Determined by Air Temps .. The Equations have grown over the years as Technologies have advanced .Yeah and the intercooler cools it right back down.
The factory intake is a hot air intake.
Any gains or losses from an aftermarket intake will be in an insignificantly small fraction of a mile per gallon.
A Good rule to follow is maitaning a most efficient combustion process over a wide spectrum of conditions .. shoot this has already been accomplished on the cruzen and would'nt any 1 want to maitain a constant efficient combustion .Even if there were gains, the amount of money you spend on an intake can never be made up at the gas pump.
Let's say an intake see 5% improvement in MPGs. That's being VERY generous. This means you would save approximately a half gallon every time you fill up. Say your intake cost $300, at current prices it would take you 200 fill ups to see the cost benefit. Thats, give or take, 80k miles.
A good rule of thumb is... Don't do mods to improve gas mileage. The gains you will see will take years to realize.
short answer -
long answer -