What is the difference between trailer tires and passenger tires. My Enclosed Haulmark came with ST205/75D15 C rated and my open 16 ft trailer has P225/70R15. Both trailers have 3,500lb axles under them. My open trailer the tires are in great shape and going on their 3rd season. The enclosed only has a few hundred miles on it and the tires are wearing like crap. Can anyone explain why they put passenger tires on an open trailer?