Some people might undercut the relevance of the wheel width to the question of what backspace to go with for the sake of simplicity, like it just doesn't matter... and a 4.5" backspace just looks good on a superduty no matter the wheel width.
If you ran an 8" wide wheel with 4.5" backspace the wheel would tuck up under the fender and totally neuder the truck's looks, not to mention try it's best to put a decent tire into the leafspring. A lot of the bigger wheels run narrower sizes, and 8" is nothing crazy.
Now on the other end of the normal wheel widths run on these trucks is a 12" wide wheel. Now a 4.5" backspace on a 12" wide wheel would put the trackwidth way to wide and make the truck look retarded for the opposite reason. Look like a fiesta ride with 14" (tall) wheels sticking completely out from under the fenders.
But even dumber than that is the fact that with the 8" wheel on the truck, the centerline of the wheel is actually a half inch Inside
the mounting flange (the rotor face). And with the 12 " wheel the actual centerline of the wheel is one and a half inches Outside
the mounting surface on the truck.
Meaning that between the two wheels options with the exact same
backspace, the trackwidth of the truck changed Three
inches just by changing wheel width.
If 3 inches is close enough
you'd best leave design to someone else because a trackwidth change of a half and inch is VERY dramatic to a truck. A change of 3 inches is ridiculous. It totally changes the way the truck looks, and functions if we even want to go there. How about the 8" wheel putting the inside of the tire into the springs, and the 12" wheel putting the tire into the fender and bumper because of the arc created by having the tire centerline one and a half inches outside the hub.
You don't just Put a 4.5" backspace wheel on a superduty
unless you just don't give a damn how your truck looks or functions.
Wheel width has got to be known. Why do they even offer all the other backspacings for 8 on 170 wheels? Must be all the other trucks that need those different spacings, lol. That or the fact that most people actually care about the way their truck turns out.