your first question is answered in this book I used to have around, it gives you the equation perfectly and then you can just put it into an expression and hey presto turning wheels... I dont have the book but I just tried this out on paper and it might work.... but dont quote me on that!

Ok let d be diametre of your wheel, pi is 3.14, m is distance travlled and n is the number of rotations of your wheel.

m = n * pi * d
so the distance travelled is the number of rotations times pi times the diametre of the wheel.

thus:

n = m / pi *d (i think that's right)

this will give you the number of rotations to make and then if you times it by 360 you should get the value to put into maya's rotational value.... in theory anyways. So you need to calculate the distance you've travelled (which is easy enough using 2d vector math)

I hope this helps I can't test it out really but if you are near a library check out some basic math and physics books and they might tell you how to do it exactly.

Alan