The Sidewall of your tire is labelled with several specifications that are important for you to understand.
Here is a brief explaination on some of the most common specification.
A. SECTION WIDTH:
Section width is the width of an inflated tire in millimeters at its widest point.
B. ASPECT RATIO:
Section width and hight are used to determine the Aspect Ratio. The lower the ratio, the shorter the sidewall.
C. RADIAL:
The "R" stands for "radial"construction.
D. RIM SIZE:
The diameter of the wheel (not the tire) in inches.
E. LOAD INDEX:
The load index number corresponds to the maximum loadcarrying capacity of the tire. Refer to load index values in the chart below.
F. SPEED RATING
The speed symbol corresponds to the tire's maximum speed capability (under optimal conditions).
Speed RatedÂ  Speed Category:  
Symbol:  MPH  km/h 
M  51  130 
N  87  140 
P  93  150 
Q  99  160 
R  106  170 
S  112  180 
T  118  190 
U  124  200 
X  130  210 
V  149  240 
W  168  270 
Y  186  300 
XR  149  240 
(Y)  186  300 



UTQG Ratings consists of the following 3 components: Treadwear, Traction and Temperature.
TRACTION
Traction grades range from AA, A, B and C (AA being the highest grade). These letters represent a tire's ability to stop on wet pavement as measure on a specified government test course. Any tire rated below C is considered unacceptable for road use.
TEMPERATURE
The temperature grades range from A, B and C (A being the highest and C the lowest). These letters denote a tire's ability to dissipate heat under a controlled indoor test environment. A tire rated below C is considered not approved for road use.
TREADWEAR
This is the wear rate of the tire, comparable only to other tires within a tire manufacturer's line. 100 is the baseline. A tire with a rating of 200 will last twice as long on the government's course as compared to a tire rating 100.