My truck came from the factory with 275/55 R20 tires. An internet search shows them to be 31.9" tall and 10.8" wide. Can a 12.5" wide tire be mounted on the same rims?

You have to look up your wheel width and then look at the approved wheel width range for the tire you want to mount. If your wheel is within the width that the tire manufacturer says, you are good. There is no way to tell by looking at the stock tire size alone.

When you say wheel are you referring to the rim?

I believe you can "model" such on Discount Tires or The Tire Rack's website.

Yes

As already mentioned, you need to know the size of the wheel. Specifically the width. It's probably 8 inches or so. Then look at the tire specs for the approved wheel width.
But low profile 20s? What kind of a truck is it?

I used the Custom Offsets website to get the info I needed. The 33x12.5x20 tires will fit my stock 2021 Ram 1500 rims but they stick outside the fender by 1". I doubt I will like the looks nor will I enjoy them throwing water, salt, and mud onto my truck. I'll wait and find a set of tires that are narrower.

285/65r20 is what your lookin for.

They're roughly 34x11 and fit most any factory wheels just fine. They typically tuck into wheel wells.

Can't believe these 20" douche-rims that come on trucks nowadays. I still prefer 16 inch factory, rusty steel wheels, before trucks were a fkn fashion statement.

275/65R20 is often cheaper than the 285 version, but not much difference in size. That’s the route I went. Pretty much a 34x11 on my factory 20” F150 wheels with small front lift.

The 2024 RAM my father just bought has 22" rims. 2024 is also the last year the 5.7 Hemi will be available so it's likely to be the last truck he buys. I'm fairly certain it will last longer than he does.

Yep, same here. 2" leveling kit and no problems.