Hi camp-n. In my case (Ford), there is no tow rating reduction with getting the optional wheel/tire package. Actually, you get an additional #100 of rear GAWR by actually getting the 20" wheels for my 2013 F150. I've got a #4050 RGAWR and I've seen where the 17" and 18" wheel F150's of the same year only have #3950 RGAWR.
Back a few years, I did see where you got a #500 tow rating hit if you got the larger diameter wheel/tire package from Ford. That is not the case now since around 2010, or maybe a few years earlier??
Anyway, in my owners manual, there is no distinction between tow ratings based on the size of the wheels. It's all about gear ratio, drive train, cab config and if you have the Max Tow option and/or the HD payload option.
I figured I'd be getting some LT tires after my first tow! But, I found that since the overall height of the tire is still basically the same as the 17" and 18" OEM tires of ~ 32" it's all about the load rating of the wheel/tire combo that matters now. My 20" wheel/tire combo is basically the same height as the standard 17" wheel/tire combo.. The diffence is the sidewall height of the tire. My 20" tires have a shorter sidewall height than the 17" tire has.
I like that... :) Not as much room for the tire to squirm around with a shorter sidewall.
Anyway, good discussion and I've learned a lot since I had an old 97 F150 with 16" wheels that I had to get LT tires because it had such a tall sidewall to make the 32" overall height that seems to be the standard "stock" height of tires coming out of the OEM.. Well, the Ford Raptor has 35" tall tires, but that's a totally different truck, right? :)
Mitch
