I like outside behind the sunroof, angled down. Set the GoPro on spot mode for exposure, and make sure the center line hits the pavement and not the car to get the correct exposure.
I can see my hands and head (LOOK AHEAD!!!). I use my iPhone and TrackAddict HD with an ELM327 WIFI obd2 unit and a Dual GPS150A Bluetooth GPS sender to get good GPS data. I use the audio from the iPhone
I also like it for autocross b/c the angle lets you see how close you're coming to the cones on each side
I've also had success with the GoPro using the skeleton back with a Micover stickover wind-screen, furry grey thing that covers the holes in the skeleton back
This angle is also useful when comparing lap to lap video as the interesting part of the video is on 50% of the screen. It's easy to crop without scaling and stretching the video
This audio isn't representative, as it's just the GoPro audio with the full sealed case on