NOTES
When the car is moving non-horizontally, things are different. The extension of the spring is no longer proportional to acceleration of the car along the road - some of the extension is also caused by the resistance of the car to rolling downhill. Unless one knows the slope of the road, one can't back that effect out, and acceleration and the quantities produced through integrating acceleration will be calculated incorrectly.
George Gamov and others considered this to be a fatal flaw for the concept of inertial navigation.