Offset describes where a wheel’s mounting face sits relative to its centerline, and it determines how far the wheel tucks in or pokes out. It’s measured in millimeters and stamped on the wheel (e.g., “ET40”).
The three cases
Positive offset means the mounting surface is toward the street side of the centerline — the wheel sits more inboard, typical of front-wheel-drive cars. Negative offset puts the mounting surface behind the centerline, so the wheel pushes outward — common on trucks. Zero offset is dead center.
How to measure it
Lay the wheel face-down. Measure the full width and find the centerline (half the width). Then measure from a straightedge across the back down to the mounting pad — that’s the back spacing. Offset = back spacing − centerline distance, converted to millimeters.
Frequently asked questions
What does ET mean on a wheel? It’s the offset in millimeters.
Positive or negative offset better? Neither — it depends on the vehicle; match the factory spec.
Offset vs backspacing? Backspacing is a distance; offset is referenced to the centerline.