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I generally try to avoid the "converter widgets" out there because if you can't "DO THE MATH" they can't be confirmed. I do the math and rely on that before commenting. The CCW though is a reliable converter, it's what they do!!
I understood your math after you posted it and I often use the "back-spacing" from a wheel/tire combination that is mounted to check clearances. In your examples you just needed to remove from the +offset to move the combination outward. The tread-width by design on the front is less to allow for steering geometry!
Good luck with your wheel search!!
I don't often refer links to "converter widgets" but here's one you will enjoy and maybe can use BUT confirm with "your math"!!!! You can mount your original combination and test "fitment". This one generally passes the "math test" I use!
I have the original window sticker to the car and it says 285/40-17 on it, and that's whats presently on the car. It is the oem size for a 9.5" wheel when equipped with the 8.5" wheel on the front (base models).
I have a 95 vette and the stock rear tires are 285/40/17.
After all that hubub about getting offset right, yours truly just bought a set of speedline magnesium takeoffs with goodyear F1 run flats.
Now instead of worrying about sticking out too far, I have the opposite problem. These bad boys are 17x8.5/60mm and 18x9.5/65mm respectively, and have 245/45-17 and 285/40-18's on them. I think I'm going to need 10mm spacers now just to put me back to stock.
Anyone running spacers with C5 wheels on their C4?