Tire speed rating system:

[McRat]

Starting in 1991, new designs were to implement a new designation system, but not all tire companies complied, so there are two systems in use with the new tire markings as the preferred system.

Old System:

225/50SR16 - 112mph
225/50HR16 - 130mph
225/50VR16 - over 130mph (unspecified test speed)
225/50ZR16 - over 149mph (unspecified test speed)

The new system has a two part designation, the first part is the size only:

225/50R16 - 225mm inflated width, 50% height to width ratio, radial, 16" rim.

the second part of the designation is a number and a letter, with the number being the weight rating, and the letter being the speed rating:

100W - 1764lb (800kg) + 168mph

So the full marking on the tire would be 225/50R16 100W, and would be the new designation for the old 225/50ZR16.

The weight numbers are:

#   LB  kg        #  LB  kg
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 

etc

And the Speed Letters are:

M 81 mph 130 km/h
N 87 mph 140km/h
P 93 mph 150 km/h
Q 99 mph 160 km/h
R 106 mph 170 km/h
S 112 mph 180 km/h
T 118 mph 190 km/h
U 124 mph 200 km/h
H 130 mph 210 km/h
V 149 mph 240 km/h
W 168 mph 270 km/h
Y 186 mph 300 km/h
(Y) over 186 mph (unspecified)

Often tires that are W, Y, (Y) will have the old ZR inside the tire size, but not always. Both 225/50ZR16 87W & 225/50R16 87W are the same.

So on tires that do not have a VR or ZR sequence in the tire size notation, you look at the Weight+Speed designation to get the tire's speed rating (and load rating).

copied and paraphrased from http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

 

[Lennart]

So which tires are you going to use at El Mirage?
W-rated? Y-rated?

 

[McRat]

V-Rated is the minimum. I try to get the highest rating I can find, but sometimes that's difficult and still be affordable.