[flybouy]

Welcome.
You have been given good advise on finding out what size tire should be on your trailer. The next step is to check the manufactures date code to see how old the tires are. The "Newest" or most recently made tire is what I'd use as a spare IF it's less than 4 years old. Age is the killer of ST (special trailer) tires as is under inflation.
From your description it sounds like you have steel wheels that are not matched. Several web based suppliers offer "wheel and tire" combos where the tires are mounted on the wheels you choose and delivered to you.
. As for tire brand you could ask what brand beer you like and you'll get fewer answers. A few brands "rise to the top" if you will on folks reporting satisfying experiences.
In the end you must make that decision based on your use and budget. You are in the right place now and if you use the search function you will have enough reading material to last you a very long time. I'd suggest you start your search with the using the correct tire size for your unit as the search terms.
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