The following paragraph is a standard/accepted tire industry quite, for the condition in the quote.
“Speed ratings are the product of laboratory testing – with simulated speeds and loads. To receive any kind of rating, a tire must demonstrate that it’s capable of sustaining a particular speed. Industry standards govern the process of reaching and maintaining a given speed during a test.”
That one is from Bridgestone.
The following reference contains, in part, the reason why we now have speed letters on our ST tires.
https://enforcement.trade.gov/frn/su...17-13287-1.pdf
The DOT symbol on the tire sidewall represents an affirmation by its manufacturer that it was tested for whatever speed letter is on the tire sidewall, if any is there. Testing is almost always done by an approved third party. Not ever tire is tested, just whatever is considered sufficient for the speed rating for that Designated Tire Size.