According to some TV show on one of the discovery/tlc/history channels, the tires on bigfoot #5 were salvaged from a one-off US military arctic snow train used to resupply NORAD radar stations. According to the bigfoot website, they were made by firestone.
I think the wheels were multi-piece, so you would unbolt the outer flange, change the tire, and then rebolt the wheel back together again. Military vehicles use the same technique to make field repair easier.