Using a P195/60R15 87S tire size as an example, the 87S at the end of the size represents the tire’s service description. A service description identifies the tire’s load index and speed rating. Service dDescriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.
 The first two digits (87S) represent the tire’s load index and are followed by a single letter (87S) identifying the tire’s speed rating. 

Load Index

 P195/60R15 87S - The load index (87) is the tire size’s assigned numerical value used to compare relative load carrying capabilities. In the case of our example the 87 identifies the tires ability to carry approximately 1,201 pounds.

 The higher the tire’s load index number, the greater its load carrying capacity. 

89 = 1,279 pounds
88 = 1,235 pounds
87 = 1,201 pounds
86 = 1,168 pounds
85 = 1,135 pounds

 A tire with a higher load index than that of the original equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the original equipment tire indicates an equivalent load capacity. A tire with a lower load index than the original equipment tire indicates the tire does not equal the load capacity of the original. 

Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 110. 

Load Index	Pounds	Kilograms		Load Index	Pounds	Kilograms
71	761	345		91	1356	615
72	783	355		92	1389	630
73	805	365		93	1433	650
74	827	375		94	1477	670
75	853	387		95	1521	690
76	882	400		96	1565	710
77	908	412		97	1609	730
78	937	425		98	1653	750
79	963	437		99	1709	775
80	992	450		100	1764	800
81	1019	462		101	1819	825
82	1047	475		102	1874	850
83	1074	487		103	1929	875
84	1102	500		104	1984	900
85	1135	515		105	2039	925
86	1168	530		106	2094	950
87	1201	545		107	2149	975
88	1235	560		108	2205	1000
89	1279	580		109	2271	1030
90	1323	600		110	2337	1060

Speed Rating

 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	U=124 MPH, 200km/h
P=93 MPH, 150km/h	H=130 MPH, 210km/h, Sport Sedans
Q=99 MPH, 160km/h, Winter, LT Tires	V=149 MPH, 240km/h, Sports Cars
R=106 MPH, 170km/h, LT Tires	Z=149 MPH, 240km/h and over, Sports Cars
S=112 MPH, 180km/h	W=168 MPH, 270km/h, Exotic Sport Cars
T=118 MPH, 190km/h	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. 

	Miles/Hour	Kilometers/Hour
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. 

Prior to 1991, the most popular speed ratings were “S,” “H” and “V.” However, while the speed capabilities of S- and H-rated tires still indicate the same speeds as before, the V-speed rating has been modified. Previously a V-speed rated tire with the “V” branded “within” the tire size indicated that the tire was capable of 130+ miles per hour as indicated below: 

	Miles/Hour	Kilometers/Hour
P225/50SR16	112	180
P225/50HR16	130	210
P225/50VR16	130+	210+

Today’s new V-speed rated tires are always identified in the tires service description: 

	Miles/Hour	Kilometers/Hour
P225/50R16 91V	149	240

The speed rating for the tires the Avenger  should have are "V", but can also use "H" and "Z"..

If using a touring radial, find one the same size.  Although the speed rating of a touring radial will not match up to a performance tire, a touring radial with at least a "T" rating can be used.

The SPEED RATING is only used to match the tires with the maximum speed of the vehicle, and that is in Europe.

Where in the United States would you go 140+ miles per hour "legally"?

I was told that if you are in an accident, and it is found that the wrong rated tires are found on the vehicle, you will not be covered.  This is not true, contact your carrier to confirm. They will all tell you, you shouldn't be going over 75 in the first place, and should use caution on bends..