RCZ

Umm...more power = more air/fuel.

Free breathing intakes can help increase the amount of air flowing into the engine. The MAF sensors pickup more air volume moving in and adjust by adding more fuel. Unless the ECU does NO adjusting at all (and the car leans out big time), adding performance parts like intakes and exhaust will reduce gas mileage at any RPM.

Think of it this way...the car is trying to maintain a certain air/fuel ratio. If you add more air, you need more fuel to maintain that ratio. This applies less at non WOT situations, but the idea is the same.

You may be pressing the gas pedal less, but you are using more fuel in those presses than you were before...In reality though. our cars are running a bit leaner, which I guess in theory means you are using less fuel. Trust me though, you are not going to get better gas mileage...

This is as much of a myth as turbo engines being more fuel efficient. They are less fuel efficient than their non turbo counterparts under WOT. The only reason why people say turbo CARS are more efficient is because you need a smaller engine to make the same power, so during the time when you are OFF boost, you are using a smaller engine and thus less gas. If you want to make 600hp from a 2L turbo you need as much air and fuel as a big NA block would.

Thats my theory.