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Tire sizes reflect the tire’s measurements in a standard format. The three measurements included in all tire sizes are:
  1. Section Width (WWW)
  2. Aspect Ratio (AASC)
  3. Diameter (DD)

Depending on their intended use, some tire sizes also include information such as speed rating.

Tire sizes are expressed in WWW/AASCDD or WWW/AACDD formats.
WWW is the tire’s section width, measured in millimeters.
AA is the aspect ratio or profile of the tire, which expresses the tire’s height as a percentage of its width.
S is the tire speed rating (optional, not all manufacturers will include it).
C is the letter indicating the tire’s internal construction.
DD is the diameter in inches of the wheel that the tire is intended to be mounted on.

Let's take a look at the ME880 280/35VR18 tire, for example.

WWW: This tire has a section width of 280 millimeters (11.02 inches). The width is measured from the widest point of the outer sidewall to the widest point of the inner sidewall when the tire is mounted on a specified width wheel.

AA: Tire sizes separate the section width from the aspect ratio with a slash. This tire has an aspect ratio of 35, meaning that the profile, or sidewall height measured from wheel to tread, is 35% of the section width. The higher the number, the taller is the sidewall.

S: Some manufacturers include also the the speed rating as part of tire’s internal construction. The speed rating notes the maximum speed a tire can sustain under its recommended load capacity. Because this rating system was created in Europe, the increments per letter are in 10 kilometers per hour:

Rating Miles Kilometers
----------------------------------
Q 99 mph 160 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
Z Over 149 mph Over 240 km/h


C: Tire sizes include a letter following the section height that is an identification of the tire’s internal construction. R indicates radial construction, the tire’s body plies "radiate" outward from the wheel’s center. D indicates plies that crisscross diagonally, used for light truck or spare tires. B indicates the tire as being belted. Belted tires are obsolete.

DD: Indicates the tire and wheel diameter to be used together - in this case, 18 inches. Some manufacturers carry tire sizes that express rim diameter in millimeters, or that include different dimensions for the inside and outside edges of the tire. That explains why the ME880 fits on a 10.5inch wide wheel.

Another important aspect related to your tire is the Balance Dot (it shows the lightest point on the tire). When installing the tire, it must be lined up with the valve stem (heaviest point of the rim).

I hope this tutorial will help you understand better what tire you should purchase next time.
 

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Nice work, sometimes we forget the basics need a home here too. Most of us had to go out and findthe basic knowledge over the years when we first came to the sport. This is a well thought-outaddition to our knowledge base.


At the same time, its also a good idea to visit the manufacturer's website for any tire you are considering. Mostpublish the tire's technical data so you can learn some things the Euro/DOT number won't tell you. Plus do a search here on RSW because we tend to post mileages and even discuss if it'll take the abuse of burn-outs etc.


BTW not all tires have a balance dot.


 
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