rubber thickness
I've seen a few flight school planes with some rough tires, cords showing and flat spots due to students locking up the brakes. The tires still work fine but look rough even though the rubber has only been on the plane for a few months or less. Our RVs have tires that stay on our rims for years, sometimes a decade.
Under regular use by experienced pilots that aren't locking up their brakes or landing with their feet on the brakes (happens), tires are pretty hardy and work well beyond what I choose to keep them on my plane. I know plane owners that take used tires and get another 100 landings on them for free, and even a pilot that takes used oil from people who change it regularly and flies another 50 hours on it for free. These planes fly fine and are in good shape, even while using up what some of us would throw away.
By the time I wear the first rib smooth, flip the tires around and wear the second rib smooth the middle ribs are close to gone as well if I've been doing a lot of paved runway landings. I like touch and goes and visit a lot of airports. Consider that UV degredation isn't as big a deal as our wheel pants cover the tires while we are airborne and most of us hangar our planes.
At 100 hours a year I elect to replace my tires after they've been rotated and both outside ribs are worn smooth. I could give these tires away and some thrifty person on grass could probably fly them for another few years without an issue, but since I travel with my RV to distant airports I elect to change them early. I don't use retreads because I'm one of those guys who would rather change my tires and carry around a few less pounds, not open up my wheel pants to accomodate the larger sized tires and like fiddling with my RV-7 as a passtime.
Maybe at the next tire change I'll cut a tire in half on the band saw to see what the rubber thickness, cords and composition looks like after I consider them worn out.
- Or -
If you want some free tires I'll be giving away a set in about two or three years when I'm ready to swap them out for a new set. I might even go for a new set of tubes too, just to know they are up to that cross country to see friends in other states.