OP if you want aggressive tires put on 20" they will need to do a lot of modification because to get a good side wall on the tire so you can deflate when going offroad, unless you take low profile 20" tires. I had 275/55/20 on my truck and then I down size to 18", well the tire I got is bigger then 20" tire but the rim is smaller. I don't know you plans with your truck but if you ever thinking of taking your truck off pavement don't do the mistake I did and had 20" it is scare driving 20" on rocks, unless you going to get mud terrain 33/35x12.5 which is a big tire. Good luck with your choice.