The offset of a wheel is the distance from the mounting surface of the wheel to the true centerline of the rim. A positive offset means the mounting surface of the wheel is positioned in front of the true centerline of the rim / tire assembly. This in effect brings the tire in to the fender well more. Conversely, 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.

Backspasing on the other hand is the distance from the mounting pad to the INSIDE of the rim..

Personally I do NOT know what you would need.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />