The treads on tires for boat trailers don’t really wear out. More often, the sidewalls deteriorate over time, showing signs of cracking in the rubber exterior. Yet even tires that look OK can fool you. After a few years, the laminates rot from within, and that can lead to blowouts. So you can’t really judge an old trailer tire by its looks. That’s why it is a good idea to replace your trailer tires every five to six years, no matter what they look like. This applies to spare tires as well. Even though they never get used, they still go bad on the inside.