The largest marking on the side of the tire is the size, load, and
speed rating. For the 2.5 RS and WRX, it looks like this:
205/55 R16 89V
The 205/55 and 16 are the size (see below). The 89 is the
load index (how much weight that individual tire can support), and the
V is the speed rating.
In smaller print, you'll also find the recommended air pressure in
PSI, U.S. DOT Compliance code, UTQG marking (see below), and various
other manufacturer's codes.
This often comes up on the forums as a topic of discussion because
of the amount of misinformation floating around. This should
help to explain what everything's about.
Tire sizes are normally listed in 3 numbers, such as 205/55/16.
The first number is simply the width of the tire in mm (205 mm).
The second number is sidewall height as a percentage of the width.
For instance, in this case, the sidewall is 55% of 205 mm: 112.75 mm.
The third number is the diameter of the wheel (rim) in inches.
So to calculate the diameter of a tire:
(sidewall X 2) + wheel
Converting the wheel diameter to mm. gives:
(112.75 mm X 2) + 406.4 = 631.9 mm or 24.88 in.
To calculate the circumference of the tire (how far the tire
travels in one revolution):
631.9 * pi = 1985.17 mm or 78.16 in.
Also be sure to check out the
Speed/RPM Calculator to
see how different tire sizes can affect a car's speed.
See Tire Rack's
Tire Tech center for an explanation of different tire size
designations used over the years.
UTQG (Uniform Tire Quality Grade)
A U.S. DOT labeling requirement for all tire
manufacturers. It has three parts: treadwear, traction, and
Treadwear is a relative designation that really
has no individual meaning. The number is only in relation to
other treadwear ratings. A rating of 150 has tread wear of 1.5
times as long as a rating of 100
Traction is a measurement of straight-ahead wet
braking. These included AA (the highest), A, B, and C.
Temperature resistance a rating that specifies
the maximum temperature the tire can withstand. A, B, and C are
given, with A being the highest.