This car draws triangular wave when it moves!
Triangular wave means that the y-component is proportional to the x-component (which is the distance traveled by the car).
The y-component is the half circumference of the middle gear and the x-component is the circumference of the car wheel. In our case, both the radii are same.
Thus, the distance traveled in y-direction is half of that in x-direction.
Therefore, the slope of the lines, tan θ = 1/2