Here is a cut and paste from the Tire Rack website. I think it will answer your questions:
P195/60R15 87S - The tire speed rating (S) is the maximum speed for which the tire is rated. For example, the “S” rating identifies speeds up to 112 mph (180 km/h).
It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer’s original speed rating, even after being repaired.
In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capabilities of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at “odd” mile per hour increments). Despite the tire manufacturer’s ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.
Speed ratings are based on laboratory tests where the tire is pressed (to reflect its required load) against a large diameter metal drum and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.
Speed Rating - Miles/Hour - Kilometers/Hour - Typical Use
N=87 MPH, 140km/h, Spare Tires
P=93 MPH, 150km/h
Q=99 MPH, 160km/h, Winter, LT Tires
R=106 MPH, 170km/h, LT Tires
S=112 MPH, 180km/h
T=118 MPH, 190km/h
U=124 MPH, 200km/h
H=130 MPH, 210km/h, Sport Sedans
V=149 MPH, 240km/h, Sports Cars
Z=149 MPH, 240km/h and over, Sports Cars
W=168 MPH, 270km/h, Exotic Sport Cars
Y=186 MPH, 300km/h, Exotic Sport Cars
*Today, the Z-speed rating is the only speed rating that is still branded “within” the tire size, as in P225/50ZR16. All other speed ratings are shown in the tire’s service description.
When Z-speed rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since that time the automotive industry has found it necessary to add W- and Y-speed ratings (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities.
P225/50ZR16 149+ 240+
P225/50ZR16 91W 168 270
P225/50ZR16 91Y 186 300
While all Z-speed rated tires are capable of speeds of 149 mph and above, prior to the W- and Y-speed ratings were identified in the service, how far above 149 mph was not identified