The concept of a rolling wheel is so simple and straight forward...roll the wheel based on the distance traveled. But to execute it is not so simple. First thing your example illustrated is that it only works on the X axis. So what if you want to travel in another direction? As you did on the Z axis, the wheel doesn't turn. What I typically do is to use a path animation. Then the wheel can travel in any direction and you can use the traveled length along the curve as your distance to drive your wheel rotation.
Another issue you will notice with the "simple" connection method is what happens when you animate the speed (accelerate). Speeding up is usually not as much of an issue as slowing down. Most people use a straight multiplication for the rotation value. So that when slowing down, the wheel will actually start rotating in reverse. Instead of a mutliplication approach, the proper way is an additive approach. At each frame, a certain amount of rotation is added. This allows for acceleration and deceleration. And for burn outs. 🙂
Now back to your original question with your specific case, you have to measure your direction and compare it to your default X axis direction. A dot product will give you a positive value if pointing in the same direction as the X axis. And a negative value if pointing in the opposite direction. Use this to multiply your rotation value.
