actually being under the hood, doesn't make it much of a cold air intake, , but,, it frees up intake resistance, and that is important,,
another also important thing to do is allow the exhaust to get out just as good as it comes in, inlet and outlet must be semi matched to allow for more horsepower,
as for gas mileage of course making it take in more air and let more exhaust out it also must use some more fuel, so mileage may suffer just a bit, but with a tune from a good computer tuner will let it use the fuel wisely, and maybe the number will increase a bit, or maybe stay the same, ,