Having worked in the Tire industry for about 4 years, I can tell you that the tires speed rating is determined by testing it at that speed for a sustained period of time. That period of time, believe it or not, is 24 hours. They (Firestone/Bridgestone at least) put the subject tire on a loaded roller and bring it to that speed and monitor it for 24 hours for things such as heat, wear, and structural integrity. If you're going to a track day, I'd suggest the highest speed rating you can find. Speed rating is also a notation of the tread compound (also wear indicator or rating depending on what you wanna call it) and generally the higher the speed rating, the higher the grip from the tires. Just a little infor for ya.
Laters,
Brian