There is a fascinating manual on vehicle mechanics called: "Mechanics of Vehicles" by Jaroslav J. Taborek. It contains a fairly detailed analysis of vehicle statics and dynamics which could be applied to robot motion. In there, the coefficient between the wheels and surface is referred to as the Coefficient of Road Adhesion.
It deals with the, "particular behavior of the elastic wheel, the coefficient of friction for rubber tires has been more appropiately called coefficient of road adhesion." p. 7
I highly recommend this as reading as you will feel smart afterwards even tho it makes no sense whatsoever
What do i know about this, i'm a CE.