If you use the water-proof back it will reduce the wind noise a little, but you're pretty much just going to have to edit in some music over the recorded audio.
The reason the videos you see online of indy cars or other motorsports vehicles have such awesome audio is because their engines are loud enough and in close enough proximity to the GoPro that the sound overpowers the wind noise. So, if you've got a stock exhaust, or even a moderately aftermarket exhaust, it's not going to be loud enough to overpower the wind noise.
Find you a cool song and do some dubbing
One other alternative is to mount it inside the vehicle. But if you do this, just remember, the audio WILL pick up anything you say, or any music you're listening to
