Courtesy of ca18guy and 240meowth here's how they are measured:
Tires are given numbers, for instance, 215/40R17 in order to measure and rate them. Here is a breakdown of those numbers.
215 = width of tire (mm)
40 = sidewall height expressed as a percentage of the width (thought this was wrong the first time, but thought I just didn't know what the hell I was talking about)
R = speed rating of tire
17 = iameter of rim it will fit (in)
Speed ratings (courtesy Bridgestone):
Symbol MPH KPH
M 81 130
N 87 140
P 93 150
Q 99 160
R 106 170
S 112 180
T 118 190
U 124 200
H 130 210
V*(VR) 149 240
W**(ZR) 168 270
Y**(ZR) 186 300
*Current tire speed rating markings include the use of the service description to identify the tire's speed capability (P215/65R15 95V -maximum speed 149 mph). Previous customs included the speed symbol in the size designation only(P215/65VR15) and the speed capability was listed as "above 130 mph(240kph)."
**Any tire with a speed capability above 149 mph(240(kph) can, at the tire manufacturer's option, include a "ZR" in the size designation (P275/40ZR17). If a service description IS NOT included with the size description, the tire manufacturer must be consulted for the maximum speed capability(P275/40ZR17--speed capability is >149mph). If a service description IS included with the size description, the speed capability is limited by the speed symbol in the service description (P275/40ZR17 93W = maximum speed 168 mph(270kph).)
**Edit**The columns got slightly messed up, but looked fine when I tried to edit them, so I don't know how to fix that, try and understand the speed ratings. <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt='
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