Got over 7 years from my original 235/80 16D US made Marathons with no issues. Starting my 4th on a set of Chinese made E rated Marathons. I swapped the 16" wheels from my old trailer to replace the 15's that came on the new one. Again, no issues. I think climate has a lot to do with tire life. I think the cooler rain forest I live in is a lot easier on tires than hotter drier climes Sidewall cracking is seldom an issue here unless tires are really old. I also never tow above 100 kph (62 mph).