You are wrong. Way wrong. More wrong than anyone has ever been about anything ever before.
Maybe not that wrong, but still wrong.
A tire is mounted evenly across a wheel no matter the width of said wheel. Therefore the tires centerline is the same as the wheels centerline. Offset is the distance between the mounting pad and the wheels centerline or the difference between the mounting pad and the tires centerline. Makes no difference how wide the wheel is. In fact you gain a little clearance at the fender running a slightly stretched tire and lose a little when running a tire too wide for the wheel since the sidewall will bulge and tends to deform when striking road imperfections.
NINJA: I just re-read your post and you are spot on if you are only dealing with the wheel measurements. Wheels don't rub on fenders, tires do.
Hah!
Ok I get it that the 7.5 wheel will stand up the sidewall with the 225/40 more than the 8 inch wide wheel and I understand offset.
Two wheels of the same offset but with different widths - the wider wheel sticks out farther. It also sticks IN farther.
Two wheels with different offsets but with the same widths, the wheel with more positive offset does not poke out as far IE the car would have a narrower track.
I'll try one more time, and forget about tires at this point.
Measured from the centerline, the 8" wide wheel is .25 wider on each side in comparison to the 7.5" wide wheel. This is because 8-7.5=.5 and divided by 2, this equals .25 inches.
When both are mounted and compared, the 8" wide wheel in this comparison is moved inward relative to the centerline by 8mm more than the 7.5" wide wheel because it has more positive offset - (45mm - 37mm) = 8mm more positive offset.
The extra width of the 8" wide wheel however is .25 inches on both the inside and the outside of the wheel. In this example we are only concerned with fender clearance so the outside is what we are looking at. Converted to mm, .25in is 6.35mm.
Still with me?
So take the extra distance it's moved inward of 8mm, and then subtract the extra distance it's moved outward because of it's additional width which is 6.35mm and you get a difference of 1.65mm
I understand the key to your point is even if the difference between how much the outside of the two rims pokes out of the car only differs by this very small amount, the fact that the sidewall isn't stretched on the 7.5" wide wheel makes a big difference in the clearance to the fender. The wider wheel stretches the sidewall and gives it a rounded profile which provides more clearance to the fender when the suspension articulates and because of how the suspension articulates.
I only had two beers, but with it I had CANDY.