The Deputy wrote: Off-set, negative or positive, is the measurement of the rims width when taken at the rim/rotor mounting surface and the inner or outer edge of the rim. Negative is from the rim/rotor mounting surface to the inner edge, positive is from rim/rotor surface to outer edge.
Brian.
Brian, I believe your definition is for back-spacing. Offset is measured as the distance between the mounting surface of the rim to the center-line of the wheel and can be a positive or negative number from this line.
Our cars utilize positive offset wheels with a relatively high offset measured in millimeters. The front wheels cause the most frustration with most years using a stock ET65 offset. It can be confusing, as in our application a larger number offset actually locates the wheel further inward, all other things being equal. A lower number offset actually locates the wheels further out. Seems backward until you think about it. As most other Porsche wheels have much lower offsets than 928 wheels, most will not work, particularly wider wheels on the front - as the combination of wider rim and a lower offset combines to "push" the wheel/tire combo out past the fender and will result in rubbing.
Here's a link to the "bible" of 928 wheels and suspension, with good definitions, illustrations, and explanation of why our cars use such a relatively high offset:
www.landsharkoz.com/forms/928wheels.pdf
