I depends which 20" wheel you have, and whether you have a 2WD or 4WD
Sport/Laramie Chrome Clad's are 20"x 9"
Painted Aluminum 20's are 20" x 8"
Not all tires labelled as a "305" are the same width, you have to go to each manufactures website
to see how wide the tire actually is when mounted on a suggested rim size.
a 305/50R20 Terra Grappler on a 9" wheel is 12.2" x 32.2"
I have those on my 10' RCSB 4x4 (20"x9" Clad Sport Wheel +19.05 offset)
Just one size up, a 305/55R20 is 12.4" wide x 33.19" tall and rubs
If you go to that same size in an LT (10ply) it's 12.6" wide x 33.07" tall (also rubs)
all three list rim width of 8.5 minimum (9.5 recommended)
My 305/50R20 clears the upper ball joint on the control arm by less than 1/4" so it works for my 4x4
(even though its lowered 2/3)
I have heard 2WD Rams are getting rub issues at full turn lock trying to run that same tire on the same wheel
I would say stay away from any tire over 12.2" wide
(based on a stock 20" chrome clad wheel)
I've heard many have successfully mounted 295/60R20 Trail Grappler on Painted 20's (8" wheel)
with a level kit up front, no issues
The tire is 12.13" wide x 34.45" tall on a min rim width of 8", 8.5" recommended