Offset
The offset of a wheel is the distance from the mounting surface of the wheel to the true centerline of the rim. The offset can be one of three types.
Zero Offset
The hub mounting surface is even with the centerline of the wheel.
Positive
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 and newer rear drive cars. A positive offset means the mounting surface of the wheel is positioned in front of the true centerline of the rim / tyre assembly. This in effect brings the tyre further into the tyre well.
Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset. a negative offset means the mounting surface of the wheel is behind the true centerline of the rim / tire assembly. This will cause the tire to stick out away from the vehicle.
Why is the correct offset so important?
If the offset of the wheel is not correct for the car, the handling can be adversely affected.