One of the most popular questions we have heard here is, "How do I measure a wheel's offset and what is the difference between a wheel's offset and it's backspacing?" At Steves Camaros, we handle a number of different types of wheels and we would like to help clear up any misunderstandings.
A wheel's backspacing refers to the measurement taken perpendicular from the mounting inner face of the wheel (the contact point where the wheel meets the brake rotor or drum) to the inner rim height.
A wheel offset refers to the distance between the mounting surface of the wheel and the rim centerline. The offset is positive when the mounting pad is outboard of the rim centerline and negative when inboard of the rim centerline. See Diagram for a complete understanding of these two important concepts.