Section width is measured at the widest point of the tire when it's mounted on the rim, so, it will not always be in the same place on the sidewall.
A 255 is listed as having an 8.5" tread width and a 10.2" section width when mounted on a 15x7.5". Now, those numbers can and will change on different width rims, but let's start there and consider your example.
On a 15x6", which is only 7" wide outside to outside, the 255 will be substantially squeezed. So, the unloaded tire height is actually going to increase because it will bow up in the middle. The tread width will decrease, because it's going to be forced to be more round. And the section will likely not be 10.2". But where is the widest part of the section width going to be? It's going to be closer to the tread, because the tread wants to resist that bending. So the widest part is going to be higher, and that is going to be a bigger issue for fender clearance unless you're running a 4x4. This is a Dart, so, the tire will absolutely be above the quarter lip and the higher you go the less clearance you have.
This tire here is on a narrower than recommended wheel, you can see the widest part of the section is pretty close the tread. This is a fairly low profile tire, so, it will look different on a 255/60/15
View attachment 1716393139
On a 15x10", the 255 will be substantially stretched. That rim is actually 11" wide at the outside lip, so, the section width of 10.2" is going to be stretched out wider. But
where is it going to be the widest? Right above the edge of the rim. There will be no nice round sidewall bulge, the tire will slope in from the lip of the rim back to the 8.5" tread width.
This tire is stretched onto too wide of a rim. But you can see the widest part of the section is right above the lip of the rim, it starts sloping in as soon as it's past the reinforced edge of the tire.
View attachment 1716393144
So now imagine the lip of both wheels above is in the same place. Do you think the clearance to the quarter changed?
Most tire calculators get section width wrong, which is why the calculator I linked above says that the tire will stick out an extra 1/2". It's just using the manufacturer specs and moving them relative to the centerline of the wheel, it's not evaluating how much the tire will be squeezed or stretched, or where the widest part will be.
For the OP, the 255's are measured on a 15x7.5 and have a 10.2" section. A 15x8" isn't much of a change, the section width will be very close to 10.2". And really the 15x7" isn't that different from the measuring rim either, so the section will likely still be fairly close to 10.2" But I'd be willing to bet that measurement for section width will not be in the same location, and the narrower rim will put it higher up the sidewall, which is not beneficial for clearance on these cars.
