Offset and backspacing are two different measurements which give you the same information. If a rim is 8" wide, the centerline is at 4". Offset is the distance from the centerline of the rim to the back of the mounting pad of the wheel ie where the back of the wheel touches the hub of the car. Offset can be positive or negative depending on whether the back surface of the mounting pad is offcenter towards the outside of the wheel or the inside.
If you laid the wheel face down on the ground, and laid a straightedge across the back lip of the wheel and measured down to the back of the mounting pad, that is backspacing. Did I confuse you--its hard to put into words, easy to see in a drawing.
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