What do you mean Otto? Backspacing and offset are the same thing, just presented relative to the centerline of the rim vs the back edge of the rim.
The two measurements are absolutely NOT the same thing.
Backspace is the distance from the wheel mounting surface to inside edge of the rim and is in no way related to the wheel's centerline.
Offset is the distance from the centerline of the wheel to the mounting surface of that wheel.
The diagram you posted above clearly demonstrates the difference. Notice the change in position of the wheel centerline relative to the mounting surface of the rim. It's crucial that the offset be within manufacturer's specs in order for the weight of the car to be carried on the spindle/wheel bearings properly.
