I am of the opinion that passenger vehicles, light trucks and trailers (yes Hay Dude that includes your 5500) should change out their tires after they pass the six year mark, although I would stretch that to seven if you aren't pushing their limits.
I made a trip out to Arkansas and only made it a couple hundred miles when a hub started getting warm. When the shop took the hub off, the brake lining fell apart. So bearings and now a brake job! While waiting on the brake job to get done, I looked over the 14 ply tires (16 inch) and that is when I noticed the DOT date code was past my comfort zone. The shop didn't have 4 matching tires so I went down the street to find four matching ones. The nearby trailer shop had plenty and he was showing me a set when I spied some that were rated at 16 ply. They were 17.5 tires already mounted with a familiar 8 lug hole pattern. I bought those and they put them on when they finished the brakes. Now my tires have more capacity than my trailer!
I really hate tire issues on the road. I went down to Trenton to pick up a FEL for my little Ford and blew one tire, which I singled out, then I blew the other one on same side 7 miles from home. So I had to pull the front one off the other side and mount it on the rear. The tires on that trailer still had the labels on them from where they were installed and had not been driven. They sat for what I guessed at to be more than 7 years, but can't be sure because the date codes don't match any I have seen. I stopped by the tire store that sold them, and they only had records for five years, but the owner remembered sending someone up there to install them. They had to stop by the Post Office to pick up a check for payment since nobody was at the farm when they installed them.
I knew better than to run those tires but figured I might get lucky since I was only hauling a few hundred pounds. The trailer had just been moved all the way to TN weighing 4400 pounds gross, so I hoped a 200 mile round trip wouldn't be an issue. I was wrong. Before it leaves here again, it will have 4 new tires but I am contemplating an axle or two change as the trailer has no brakes on either axle. I might go with 7k axles so I can run 16" tires with 8 lugs. That might be a little bit too much for that trailer, lol.
David from jax