If you run them loaded... near max capacity, they will fail and you can expect to loose a tire almost every trip, if not every other trip. We haul hay several times a year and run at max load or sometimes slightly over. And we were running " what ever" was on the trailer, AND yes, every trip was an adventure. On my 40ft gooseneck I switch to the 6 year rule, and had almost no failures running over loaded... Then I went from E rated to G rated.... and have had NO failures on tires since. My brother is still running the " what ever" E rated tires and we(he) have still have a blowout on his almost every single trip. Some of them are 10 years, some are 13 years old. And,, its always 106 degrees, a super busy 2 lane highway, a bad incline, and his tools/jack dont work. So, if your going to run a "fair" load or more, you really need to replace the tires. If your hauling a riding lawn mower, then no, you might not need to change them out. Always carry a 20 ton bottle jack and 2x6 blocks, to lift a fully loaded trailer. A common 3 ton floor jack is worthless.