How tires are measured/rated

[DrDubbleB]

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=''>

[AKADriver]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (DrDubbleB @ June 26 2002,9:29)</td></tr><tr><td id="QUOTE">40 = size of sidewall of tire aka profile (mm)</td></tr></table><span id='postcolor'>
This is incorrect.

The second number is the profile, but it is not just a sidewall height in mm. &nbsp;It is the sidewall height expressed as a percentage of the width. &nbsp;A 215/40ZR17's sidewall would be 215 * .40 = 86mm tall.

[sil80]

I asked a question on FA that I think belongs in this thread... gives a little detail.

Question -
Why would someone run larger tires up front? I have seen this a couple times on S13's.

16 inch JDM Buddy Club P1 wheels, rear 8.5 inches 245/45, front 7.5 inches 225/50 Sumitomo HTR-Z tires

Answer -
You have to realize that tire profile isn't an actual physical number -- it's just a percentage. The sidewall profile represents the height of the sidewall as a percentage of the tread width. So a 225/45 tire doesn't have the same height sidewall as a 245/45, even though they have the same profile.

To answer the question -- it's done because that's often the closest match in sizes between front and rear.

In your example -- a 245/45 has a 110.25mm tall sidewall, whereas a 225/50 has a 112.5mm tall sidewall. The next lower size for the front, 225/45 has a 101.25mm tall sidewall. The 225/50 is closer in diameter.

Asad