Ok, lets get the definitions straight first-
Backspace- On the inside of the rim (car side), the distance measured from the outside lip of the rim to the wheel mounting surface. You can measure this by laying a straight edge across the outer lip and measuring to the wheel mounting surface
Offset- The distance between the centerline of the rim and the wheel mounting surface. Offset can be positive or negative, positive offsets move the wheel mounting surface "out", so, more backspace. Negative offsets move the wheel mounting surface "in", so, less backspace.
Here's an illustration. Nevermind that I modified this for an 18" wheel, it applies to any diameter 9" wheel. This one is a +25mm offset, which is a 6" backspace on a 9" wheel. A zero offset 9" wheel has a 5" backspace.
View attachment 1715111111
The different measurements are nice to have, because of how things change relative to the measurement. If you go from a 7" wide wheel to an 8" wide wheel and you want the centerline of the wheel to remain in the same spot, you keep the same offset because the offset measurement is independent of wheel width. But if you use backspace and change the wheel width the backspacing must also change to keep the centerline in the same place.
So, lets say you have a 15x7" with a 4.25" backspace, and replace it with a 15x8" with a 4.5" backspace. You have more backspace, but you actually lost clearance on the outside (fender side), not the inside. You can see that with the offset. A 15x7" with 4.25" of backspace is a +6 offset. A 15x8" with 4.5" of backspace is a 0 offset. So, the center of the rim actually moved a 1/4" toward the fender, even though the backspace is more, because the width of the rim changed too.
Your measurements are pretty helpful. So, with a 15x7" and 4.25" of backspace you have 1" to the springs and 1/8" to the quarter. You need a 1/2" to the quarter, minimum, if you don't want any rubbing. And you only need a 1/2" to the springs, so, you have plenty of room. So you need at least a 15x7" with 4.625" of backspace (4 5/8"). That's a +16 offset. And that's the bad news. The 15x8 with 4 5/8" backspace is only a +3 offset, so, it would actually be WORSE than the 15x7 +6 you have now. And the section width of the tire would probably be wider on the 8" rim, so you'd lose even more space.
If you go 15x7 with a 4.5" offset, it will be close. You only get an extra 1/4" to the quarter, so, by your measurements you end up with 3/8" to the quarter. That's not really enough. It might work most of the time depending on your ride height, how many passengers you carry, how much stuff is in the trunk, or how enthusiastically you corner. But it will probably rub under some of those conditions. What you really need is a 15x7 with 4.75" of backspace. That would give you a 1/2" to the springs and 5/8" to the quarter, which would be great. But that would be a custom rim.
