Join Date: Feb 2011
Location: Colorado Springs, CO
I sell tires as part of my business. While it's called a speed rating, I tend to refer to it as a PERFORMANCE rating. The recommended performance rating is on the car and manual, and it's generally not recommended to use anything below the factory rating. Many tire stores won't do it due to liability reasons. The thought is that the suspension was engineered with a particular rating tire in mind. Can you do it? Sure you can, the downside is the car may not handle as designed. Have I done it, yes, results, some passable, some crap.
While the speed is one part of it, the meat of it is the performance characteristics of the tire. Characteristics such as sidewall stiffness, durability, tread deformation, and more are all part of the equation. The only real variable that the public may understand of all this is speed rating. Construction of the tire leads to the speed rating. Speed rating is just misleading, and just comes across as marketing.
Tire Rack has an awesome article on speed rating you should check it out.