GForce Drive is a gravity force visualizer for your car/moto/bike drivers, who want to keep track of the g-force that the driver is subjected during acceleration, breaks and stearing.
Also when the vehicle stops it works as a clinometer.

You simply have to put the smartphone standing as vertical as possible in your car, set the G scaling parameter (by default 1G, enough for normal drivers who are not subjected to forces grater than 1G) and start driving.