View acceleration, braking and lateral G forces; speed, air temperature (internet connection required), 0-100 km/h (0-60 mph) acceleration time and maximum recorded G force while you drive your car.
DroidDrive will display all those things on a display which resembles Nissan GT-R's electronic dashboard. You just need to fix your phone somewhere on the dashboard so it won't move around and it will be easy to see, or even on your windshield (with proper mounting device).
Internet connection will be required for temperature readouts (data charges may apply). Also, GPS reception is required for speed measurements.
WARNING - while you're driving, you should pay attention to driving and nothing else.