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Yeah, that's a really old article that no doubt has some errors in it.
Better terms are "offset," which is how far the center of the rim is placed inboard or outboard of the wheels mounting surface (inboard is positive offset), and "backspacing," which is how far the edge of the rim protrudes inboard of the mounting surface.
Theoretically, you want to keep the offset stock so the center of the wider tire remains where the center of the original tire was. Backspacing should therefore increase by 1/2 the amount of width increase -- if you go from a 4-1/2" wheel to a 6" wheel, you want to increase the backspacing by 3/4".
Unfortunately, I don't have good specs on how this stuff measured out on the various OE wheels. In actual practice, you can get away with just a bit less positive offset than stock -- too much less hurts the handling.
--Phil
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