IF there is any real de-rating for 20" tires I think the reason would be the load rating of the tires.Then throw in what tire size can do (17" vs 20")
I've been trying to decide on right combo, and came across this:
Tow Ratings for Dodge Ram 1500 with 20" OEM Wheels
Still trying to figure this out. At first I was thinking 17" and 3.55, but I do like those 20" filling out the wheel well--so I might have to think 3.92.
I'm not sure what load range is for the 17 and 20's but the 17's are available up to load range E.
Also one other item to consider is the limited choices in 20" tires when it comes time to replace them. I know I can replace my 17's for about 2/3'rds the cost of my dads 20's.