ok what i've learned:
make sure the tires you buy are rated for the load capacity of the vehicle or trailer. So for instance since my trailer is 21,000 lbs gross. My 6 tires each need a minimum single axle load capacity at 3500 lbs. Now the tires need to be "LT" or "SP" in order to achieve the full rated capacity. Passenger car tires or light duty tires are not allowed to be used at 100% load capacity. This is why almost all trailer tires are "SP" (special) or use "LT" which comes standard on heavy duty trucks.
Ok so lets say you found a tire that is rated for 3500 lbs, now you have to look at "PSI" the tire has to be at to obtain that load rating. Some tires say 80 psi for a max load of 3500lbs single axle. But some say 100psi for a max load of 3500lbs single axle. Therefore, whatever rims you put the tires on have to be rated for that psi. All rims will have a psi rating stamped on either the back or front. Quality rims will have the stamped marking at or near the valve stem.
Ok so now lets say you have a trailer that has 4 wheels per axle, as opposed to just 2. The load rating for the tire is different. Most of the time the tire will have a stamping on their for single and dual wheel. The dual wheel max load rating is always less than the single. This is due to the fact that the tire manufacturer takes into account spacing between the tires so rocks and small children don't get stuck. Therefore, you have to make sure you use the lower max load rating.
wow, my brain hurts. P.S. all of this information can be obtained from tirerack.com in the technical section.
I suggest you do your OWN research rather than rely on a tire shop, they just want to make the sale and get the next customer thru. The DOT will not care where or who installed the tires, if your stuff aint up to snuff, guess what, your paying the ticket not Discount tire.