You need to calculate the angle between two points along the curve (say, the lower edges of your "car"), the formula would be this:
function getAngle(p1, p2) {
var deltaY = p2.y - p1.y;
var deltaX = p2.x - p1.x;
return Math.atan2(deltaY, deltaX) * 180 / Math.PI
}
Here is a rough working example with your code: