Tire\wheels

That^^
And 255/25.4 x .7 = 7.027 checking width. And at the bottom of the tire-page is the +/- 1 inch proviso; so the rim can be 6 to 8 inches. Thus 8 is a natural for 255s.

The section width will change when the tire is mounted on a rim size other than the checking rim. I have no spec for that , plus it's a moving target.
I would guess the change is small. Perhaps the 255 section width,on an 8incher might be closer to 265, or 10.43inches.