Measure out from your current rim edge, to where you would like the new rim edge to sit. Add that to the existing width of your wheels. Then you would need to calculate the offset of that imaginary wheel.
Draw out the wheel in section, so you would have a line say 7" long, representing the width of the wheel, and mark the center as a vertical line, measure the back space of the wheel you used for the test measurement and draw that on as a vertical line as well. The difference between these too is your offset. Compare those specs to what is available "off the shelf" from weller.
You could even put the specs of this imaginary "perfect" wheel into something like
www.willtheyfit.com and use that to compare other available wheels.
Dont forgeth that you can lower the offset of higher offset wheels by using spaces, but you cant increase the offset of low offset wheels.
If you decide on a width then basically if the offset is lower (a low positive or negative number) it will push the wheel out from the car, if the offset is higher it will pull the wheel into the car and more under the arch.
You could use your diagram of your perfect wheel and compare this to simmilar diagrams of wheels available - that might help with comparing different widths and offsets to get the right combination.
Presumably you have also followed other build threads - RTLkyuubi's for instance and seen how wheels with know specs fit on other cars the same as yours.
Have my ramblings been any use at all?
(maybe 8 x 13 on the rear ?)
