So what speed do you want to drive with your trailer?

 I see many folks claiming their ST type tires have "speed Rating well over 75 mph.

They are basing this on the "heat Resistance" test that runs tires at 88% of the max load for 10 minutes steps up to the "Goal" speed. If a tire can run 10 minutes at that speed the tire is said to be "speed rated" for that speed.

I do not recall seeing any advertisement for ST type tires that mention speed mentioning "at what load"

Here is the info directly from US Tire & Rim Association. 

Notice the adjustments for inflation and load used to allow a tire to run faster than 65 mph.