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This is the distance from the back edge of the wheel to the hub mounting surface. To determine the wheel backspace:
Position the wheel face down.
Lay a straight-edge across the back of the wheel.
Measure the distance from the straight-edge to the wheel's hub mounting surface.
The wheel's offset is the distance from its hub mounting surface to the centerline of the wheel. To determine wheel offset:
Position the wheel on a flat surface and measure its overall width.
Divide the overall width by two, then subtract this result from the backspace value.
Offset=Backspace - (Rim Width ÷ 2)
The offset of a wheel can be one of the following three settings:
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front-wheel drive cars.
The hub mounting surface is toward the back or brake side of the wheel's centerline. "Deep dish" wheels are typically negative offset.