I can't speak to 20" tires, and perhaps there's a difference, but my experience with both car and truck tires up to 18" is, the few times I've chosen other than Michelins, I have been very disappointed with getting more than 30,000 miles.
My most recent set of Mich truck tires went over 60,000 miles. However, and it's a big however, I had a RR blowout while pulling my FW last summer. I don't know if it was caused by a road hazard, age, or miles. It was the first Mich blowout I've ever had. Regardless, I still went out and replaced the whole set with more Michelins.