The_Family_Tahoe
I'm Awesome
The critical measurement that determines interference in the front is the amount of wheel that hangs out over the wheel mounting surface. The maximum amount is roughly 4 inches. Some say 4.25, but I've seen 3.75 contact the fender lip. So, roughly 4 inches. This measurement will reduce if a 12 inch tire is mounted on an 8 inch wheel, because that additional tire width reduces clearance, so for this example you need to keep the tire width within 2 inches of the published wheel width. For this to make sense I'm going to walk myself through standard wheel measurements and create an example:
I'll use a 20x8 inch wheel as the example because there are lots of wheels available today in this size. Wheels are one inch wider than the published measurement. The bead is about 1\2 inch on each side of the wheel so if we include that part of the wheel on an 8 inch wheel the total measurement is 9 inches wide. An 8 inch wide advertised wheel with 4.5 inches of backspace is zero offset. Backspace is measured from the wheel mounting surface to the very inner edge of the wheel. Zero offset means the wheel mounting flange is centered in the wheel. On an 8 inch advertised wheel 4.5 inches of backspace is equal to zero offset.
If I were to bolt this theoretical wheel on a gmt400 the wheel would be 1\2 inch too far out and the tire would hit the fender lip. It's only 1\2 inch too much, so rolling the fender lip would possibly save the tire from getting shredded by the fender lip but it might still rub even with fender rolling. It certainly would interfere with wide tires, tires wider than 10 inches would 100% rub in this scenario, even with rolled fender lips.
There are options to clear the fender with an 8 inch wide wheel without rolling fender lips. An 8 inch advertised wheel with 5 inches of backspace will clear, or narrowed front control arms could be used to bring the wheels in to provide for wider wheels and tires.
Keep in mind the alignment will play a role in clearance. I personally had a stock wheel and tire combination get hung up in the front fender lip and ruined a brand new Michelin tire because the alignment was off. It wasn't far off when this happened, the truck went straight down the road if you let go of the steering wheel. After a proper alignment it never did that again. I guess my point is these measurements can be used as a benchmark but are not written in stone to apply to all gmt400s. I've listed what worked for my rig above and will summarize again below.
I run the IHC control arms with 31 inch tall 255/55r20 tires 30x10 inches on 20x8.5 inch wheels with 5.25 backspace. The IHC control arms are narrowed .75 inches per side which in theory provides 1\2 inch of clearance. This wheel has 4.25 inches hanging outboard of the wheel mounting surface. Used with the .75 inch narrowed IHC control arms they provide 4.25 - .75 = 3.5 inches of clearance which is 1\2 inch less than the 4 inch maximum. This combination clears the front fenders and the fender lips are not rolled. In practice with the 255 width tire there is NOT 1\2 inch of clearance between the top outer edge of the tire and the fender lip. I can't fit the width of my finger tips between the tire and the fender lip. It's close. I suspect the reason it doesn't rub is because as the suspension compresses and the wheel moves up into the inner fender well the top of the wheel also tilts inboard because the upper control arm is shorter than the lower control arm.
I don't think I've directly answered your question yet. If you want to run a 9 inch wide wheel in front it can be done relative to the control arm width (in theory) with the following backspace requirements:
Stock control arms and 9 inch wide wheels with at least 6 inches of backspace or +25mm offset
IHC control arms and 9 inch wide wheels with at least 5.25 inches of backspace or +6mm offset
CPP control arms and 9 inch wide wheels with at least 5 inches of backspace or 0 offset
Stone Custom Fab control arms and 9 inch wide wheels with at least 4.5 inches of backspace or -12.7mm offset
You can see from the list above the backspace gets more realistic for common wheel offerings with the 5x5 bolt pattern as the control arms get narrower. In other words there is a larger selection of off-the-shelf wheels that will fit if narrower control arms are used.
If you decide to use an 8 inch wide wheel the number of wheel choices go up. The most common backspace for 20x8 is 4.5 or 0 offset which, again, in theory, offers .25 inches of clearance with the cheaper IHC control arms. I'm running more theoretical clearance than this with IHCs and it is super close between the top outer edge of the tire and the fender lip. If it were me running a 20x8 with 4.5 inches of backspace I would not use the IHC control arm and instead use the CPP or Stone Custom Fab.
Others, please point out if I've missed something or goofed up some math somewhere.
