I'm 80% sure you'd lose volumentric efficiency, meaning you'd actually lose MPG by decreasing fuel and air intake. You're better off improving mpg, by reducing the vehicle's weight, doing proper tune ups, keeping your tires inflated properly, stock wheels, stay away from full throttle, good spark plugs and ignition rotor, keeping your ignition timing at specs, removing the backseat if you don't need it, and driving without the A/C on.