Back space help please!!
I had to think about it more and edited my response. The outside edge of the wheel will be in the same location with either wheel. The tire will just fit a little differently on the new wheel, which may or may not cause a clearance issue.
no, it won't, because were the wheel mounts to the car is in a different spot on the wheel. one mounts at the centerline of the wheel, the other is 1/2" inboard
drawing poorly in ascii the cross section:
8" wheel, 0 offset
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7" wheel, negative offset
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aligning the wheel mount face (where it attaches to the car), you see the outer sidewall of the tire is pushed out farther, even with the same "frontspace".
look at it this way. a 255 tire is 10" wide. on a 4.5" backspace, 8" wheel (0 offset), 5" of the tire is to the inside of the wheel mount face, 5" to the outside. on a 3.5" backspace, 7" wheel (-1/2" offset), 4.5" of the tire is to the inside of the wheel mount space, BUT 5.5" IS TO THE OUTSIDE
this is why looking at offset, rather than backspacing makes figuring out tire fitment so much easier from a wheelwell positioning standpoint..... a 5" wide 0 offset wheel will put the tire in the same spot as a 7" 0 offset wheel as a 10" 0 offset wheel, but the backspacing on those 3 widths are 3", 4" and 5.5" respectively.