Sierra Club

Found this explaination on the sticker of a new brand of tyre at work the other day. It's a good explaination of what it all means.

Treadwear
The treadwear grade is a comparative rating based on the tyre when tested under controlled conditions on a specified government test course. For example, a tyre graded 150 would wear one and a half times as well on the government test course as a tyre graded 100. The relative performance of tyres depends upon the actual conditions of their use, however, and may depart significantly from the norm due to variations in driving habits, service practices, and differences in road characteristics and climate.
**This would also tell me that the lower the number, the softer of a compound it is likely to be and therefore stickier, and vice versa**

Traction
The traction grades, from highest to lowest, are AA, A, B and C, and they represent the tyres ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. A tyre marked 'C' may have poor traction performance. WARNING: The traction grade assigned to this tyre is based on braking (straight-ahead) traction tests and does not include cornering (turning) traction.

Temperature
The temperature grades are A 9the highest), B and C, representing the tyre's resistance to the generation of heat and it's ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperature can cause the materials of the tyre to degenerate and reduce tyre life, excessive temperature can lead to sudden tyre failure. The grade C corresponds to a level of performance which all passenger car tyres must meet under the Federal Motor Safety Standard No. 109. Grades B and A represent higher levels of performance on the laboratory test wheel than the minimum required by law. WARNING: The temperature grade for this tyre is established for a tyre that is properly inflated and not overloaded. Excessive speed, under-inflation or excessive loading , either separately or in combination, can cause heat build up and possible tyre failure.

Another few pieces of information:
Say if a tyre size is 175/70R13 82T, the 175 is the width in mm of the tyre, 70 is the profile or height, this is expressed as a percentage of the width and the 13 means it is a 13inch rim. The 82 is code for the load the tyre can take, which will be written on it in KG or can be looked up. T is the speed rating, which beyond it, the tyre may fail. However, if everybody drives legally, no tyre will be even near it's maximum speed! Also, in some cases, there can be writing after the size, like this: 175/70R13 82T M+S, this just means the tyre is suitable for use on Mud and Snow.

Here's a brakdown of speed ratings, most tyres now are at least 'S', but generally 'T' upwards. (These are in kmh)
N - 140
P - 150
Q - 160
R - 170
S - 180
T - 190
U - 200
H - 210
V - 240
Z - 240
W - 270
Y - 300
VR - 210+
ZR - 240+

Some top info there Paddy, was never sure what the treadwear grade actually meant in real terms!