Guide to Wheel Offset or Backspace
Backspace - ET - Offset - what does it all mean ?
Put as simply as possible, wheel offset (ET) is the distance, expressed in millimeters, between the mounting plate — the surface of the wheel that mates against the rotor hub when installed — and the centerline of the wheel — the line running around the wheel’s barrel, which determines the center of its width. Positive offset means that the mounting plate is in the outside half of the wheel, toward the wheel face.
Most OEM wheels, and most wheels in general, will have a positive offset. Negative offset means the mounting plate is on the other side of the wheel’s centerline, farther toward the suspension.
Negative offset is generally seen in very deep-dish or deep convex aftermarket wheels. The particular offset, therefore, determines how the wheel will sit in the car’s wheel well, and how much of the barrel will extend toward the suspension. While most aftermarket wheels are made in multiple offsets, most OEM wheels only come in the offset that is correct for the specific car for which they are made. Closely related to offset is the concept of backspacing. Offset and backspacing are often spoken of as if they are the same thing, but they are very different.
Backspace is defined as the distance between the mounting plate and the inner flange of the wheel. Backspacing is, therefore, the combination of the offset and the wheel width. This comes into play if the new wheels are wider than the old, as the offset may need to change to compensate for the greater width.
Funfact: ET stands for the German word "einpresstiefe" which means "offset"