i saw the size trailer tire i need (155 80 r13) at wal-mart for $35. they are passenger car tires where i have st tires. i'm tempted to try them for that low cost. has anyone tried wal-mart car tires on their trailer?
What you need to look at is the load capacity on the sidewall of the tire. If it is less than what your trailer tires indicate you are asking for trouble, especially if the rig weighs 75% or more of the total capacity. In other words, just because the existing tire and trailer tire are the same size, does not mean they can carry the same weight. That's where the "Load Range" comes into play.
I have to agree that so long as the weight rating is not exceeded a passenger tire should be fine. While all my boat trailers have ST rated tires, I have had a set of passenger tires on one of my utility trailers for 10 years now with no issues. Plus, I have several pickup truck bed trailers which have been running the tires that were on the truck they came from for years.
The biggest advantage to trailer tires is the heavier sidewall and in most cases less rolling resistance. The heavier sidewall also gives them a higher load rating.
With heavier loads, passenger tires, with their softer sidewalls, can exhibit some sway where as the appropriate trailer tire will not. This will not likely ever come into effect if your towing an 800lb boat around.