Post on an RV Advice forum:
"We have ST rated tires. Do we have to run less than 65 mph? We see a lot of RVs passing us."
"Speed Symbol" is actually a relative measure of heat resistance.
The speed number is based on a brand new tire being able to run for 30 minutes on a perfectly smooth steel drum, at a constant speed without coming apart. Any tire running the test is considered scrap after the 30 minute run.
65 mph is the assumed Max operational speed for ST type tires.
This is part of the calculations used to establish a tire load capacity. While it is unlikely that your tires will come apart as soon as you run 66 mph, it is true that the faster you run the "faster" you are using up the life of your tires. People running faster than 65 is one of the contributing reasons for ST tires to have shorter life than tires on your car or Pick-up.
I often compare tire speed rating like your engine "Red Line" speed. If your car indicated 4,500 as the Red Line you can probably run 4,495 RPM for some time without the engine blowing up. BUT I would think you know this will significantly shorten the life of the engine. Same with tires.