Again, I think you misunderstand. Mostly because you're using inches to refer to offset, which is typically listed in mm.
Yes, the centerline of a 8" wheel with a -25mm offset is in exactly the same place as a 7" rim with a -25mm offset. I used the proper terminology, a -25mm offset on an 8" wide rim is equivalent to a 3.5" backspace. However, a 7" rim with a -25mm offset does not have a 3.5" backspace anymore, it has a 3" backspace. That's why it's important to use the correct terminology. Backspace and offset are not the same. Keeping the offset the same keeps the centerline of the wheel in the same place regardless of width, keeping the backspace the same does not keep the centerline of the rim in the same place if you change the width of the rim.
But don't take my word for it, here's the math. Same offset as your example, -25mm or 3.5" for an 8" rim. Same tires. Notice the clearance. The centerline of the wheel hasn't moved, so all the clearances are the same. The backspace however
has changed for the 7" rim, even though the offset is the same.
View attachment 1714985761
And now, the same exact combination, but with the backspace held constant at 3.5" for both. Notice the offset is now different for the 7" rim, and the centerline of the wheel has moved (the wheel is now closer to the suspension for the 7" rim).
View attachment 1714985762
