Based on the 2016 source book, the 18" and 20" wheels are rated to 3590#, 17" wheels are rated to 3195#.

In the commercial order guide there used to be a code for a real axle GAWR de-rating requirement to option 17" wheels to a SRW pickup.

18" wheels are the base size for max GAWRs on SRW trucks, hence why Ford started offering an 18x8" steel wheel for XL F-350 SRW trucks, to increase the RGAWR over the limit imposed by 17" wheels and tire load index.

Only 17" tire options are rated in the 3195# max capacity range. 18s are closer to ~3600# and 20s are ~3700#.

The max sidewall rating is one thing, but normally the tire inflation sticker will run lower pressures to match the load index table of the tire with the GAWR. This is one example of how people will argue about what the "correct" tire pressure is, complete with pictures of their tire stickers to prove they're correct, when in reality they're all correct depending on the FGAWR being upped by +1 or +2 spring packages. The recommended inflation pressure will vary to cover that lower/higher rating on otherwise identical trucks. Some trucks say run 55psi front/75psi back, others say 75psi/all. Lots of variables to consider.

But tire load capacity isn't the only factor. I ran 295/70R18 tires rated to 4080# each, they still wrapped around wheels rated to 3525#. (model year 2007)

 

You obviously haven't looked at the load ratings of 20" truck tires

 

Mine are Cooper AT3.........but have seen a lot of them at 3750.

 

good move, I swapped out my factory 20's for a set of Method 18's

 

Don't agree with this. My '16 came with 17's from the factory. How would back spacing effect the wheel fitting over the brakes?

All things equal, a larger wheel will always have a higher load rating. Thats the physics of a circle. In my opinion, tire choice are better for 20's than they are 18's, but neither are limited this day in age. And load ratings for 20" tires are usually much higher than the vehicles gvwr when the correct ply are chosen.

 

It's been an issue that some aftermarket 17" wheels will not clear brake calipers, but as far as I know this has been a problem since 2005 when the base rim size stepped up to 17", not ~2014. I don't know that backspacing really plays into it, if a wheel would rub a brake caliper if it were set in another inch, probably shouldn't run those wheels anyways, that's not even getting into the problems these trucks already have with unit bearings without spacing the wheels out a few more inches.