OK I had you going with the previous Tutorial.
It is the mathematically correct formula to make a wheel spin. But it can be simplified.
As you can see in the Tut I use Pi twice, one to multiply and once to divide with, and that is useless.

Follow up:

Ok here we go again, take a spline and a cylinder in a Null. Align the null to the spline with the align to spline tag.
Drag the spline, the cylinder and de tag in the xpresso editor.
Now add a spline node and connect the spline to its object port and open the length port. Divide the result by the position of the Align-to-spline tag. The divide the result by the radius of the wheel (cylinder).
Connect the result to the rotation of the wheel.
That is it really !

The hardest thing is connecting the two images correctly