There are tons of military surplus tubeless tires out there, mostly Michelin’s but a couple other brands as well.
Are they 20" or 22.5"? I've never seen a 20" tubeless HD truck tires...
You can also look for 6-lug Budd to Dayton Rim Adapters and then run 22.5 Tubeless Dayton's...or your 20' tires on Dayton's.
Mike B
