**Check your axis.**

Recently answered [this question](https://blender.stackexchange.com/questions/209027/bone-driver-on-x-location-not-working/209299#209299) re bone drivers. 

[![enter image description here][1]][1]

In the armature (data) properties viewport display tab, turn on Axes. Or use translate tool and put in LOCAL transform orientation mode. Your screenshot is in GLOBAL.  This will show the directions of the local axes. For bones Y is head to tail. 

[![enter image description here][2]][2]
_Bone on right is root bone, when moved locally in X, rotates other bone in Z._

[![enter image description here][3]][3]

Mapped from Local X location (0, 1) to Local Z rotation (0, 360), with Z source targeting X 

_Your axis alignment may differ, can only tell Y from screenshot. Change to suit, but have feeling your wheels will spin if you move car sideways 8)-._

Note, if you know the diameter of your wheel (d),  move pi x d for every rev. to make rotation look real.  Generally for wheels would spin a bone on its Y axis, but it's arbitrary.

  [1]: https://i.sstatic.net/QOgj6.png
  [2]: https://i.sstatic.net/oGD3F.gif
  [3]: https://i.sstatic.net/c5nlB.png