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Wheel Backspace
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.
Wheel offset
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:
Zero offset:
The hub mounting surface is even with the centerline of the wheel.
Positive offset:
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.
Negative offset:
The hub mounting surface is toward the back or brake side of the wheel's centerline. "Deep dish" wheels are typically negative offset.