20" wheels...reducing tow capacity?

Well with your tire size, I'm assuming you have "P" style tires versus LT. My truck came with 275/55R20's stock as well and is an F-150. I recently bought new tires but I decided I want to get an "LT" style tire since I didn't like the squirminess of "P" style tires when hauling dirt. So I went up a few sizes to 275/65R20 since that's a stock super-duty sized tire and they're the cheapest LT tires I could use without having to buy a new set of smaller rims. I've also always liked 35" tires on my truck and only reduced my effective rear axle ratio from 3.73 to 3.55. When the OEM's spec'd the tire size, most tires in that size have the same load rating, so payload capacity wise you should be perfectly fine. The nice thing of the 275/55R20 P series tire is that they do ride softer when unloaded. I noticed that my truck rides a bit firmer having to run higher tire pressures to maintain payload capacity, but I actually prefer that on my rides.