Id like to run a 33" tire, and try to keep them under the wheel wells. If the tread is 12.5 or 13.5 how would the inch or so rim size differ? the tread width is the tread width right?

If your looking to keep them under the wheel wells, the 8.5 would be better for you. The minimum suggested rim with for my 13.5 wide toyo's is 8.5. I have them on 8" wide rims and have to air them down a good bit to get a good contact patch. It might be best to stick with 12.5's on 8.5 rims. Check with the tire manufacturer your going with and it should list what the acceptable rim width's are.

to keep them under the fender wells you need to run 17/8 wheels

I have 17x9's with the stock flares, they are under.

usually the width given is the overall width of the tire, usually found about halfway down the sidewall, unless specifically stated by maker. 12.50/33 tread width is around 10.5", 14.5/33 tread is about 12.5-13.5 +/- a bit.

to keep them under the fender wells you need to run 17/8 wheels

Are they still under a good bit? I am wanting to go with 16 X 8 with 285's....but I have been holding back because I was afraid they may stick out.

my 16x8s w/ 4.25" BS stick it just a tad with 285/75R16 MT tires.

I think I have a shot down the side of truck in my garage for reference to this.

C-ya