The basic form of the equation you're going to need is:

degrees rotated = (360 x distance traveled) / (2 x pi x radius of wheel).

The general form of the Expression you're going to need will be'

wheel.rotateZ = (360 * wheel.translateX) / (2 * 3.1415 * [radius of wheel]);

Now, obviously, I have no idea what the radius of your wheel is, so you'll have to find that out yourself. And pi can be used as '3.1415' or to any number of other places (although I think there's a way to summon the exact value in maya, I can't think of it at the moment - and in most instances you're probably not going to need anything more than the 4 digits or so.).
Also, depending on how you have the rig set up, you may have to change what the wheel uses to grab its translation (for example, if you have the wheel parented to something, moving the object won't actually change its location - it'll be at a local '0'). Just be aware of things like that.

That help?