Since the wheels are parented to something else, they aren't actually translating in the scene. The parent object (or its parent, etc) is translating, and the wheels are just following in the parent object's coordinate space. To make the expression work, it needs to get the translation of whatever is translating everything. If it's the body of the car, then use that. If it's a group object, use that.

BTW: This isn't really a newbie/basics question. This should go into the programming forum.