Why a tire with better UTQG traction rating, AA versus A and the same temperature rating of A, is rated at lower speed: H versus W ?

Yokohama Avid H4S has UTQG of 500 AA A but speed rating is H
Fuzion ZRi has UTQG of 320 A A but speed rating is W

What wrong with the data ? May be Avid H4S temperature rating is at A- and ZRi is at A+, therefore ZRi can be driven at higher speed without exploding ?

 

TR-S2K

The 3 digit number refers to a tread wear index, not a traction rating. Traction ratings (the AA and A) are only comparable within a speed rating (like H, V, W, Z etc.) The speed rating is proportional to the heat the tire can withstand at speed without failure (among other things). Remember, though, that's with a tire under its max loading...if you overload a tire, or underinflate a tire you will generate more heat at speed, and perhaps cause failure. I think that UTQG stands for uniform tire quality grading (guys help me out if I'm incorrect), but tire performance and characteristics are anything but uniform. This is where the survey section and pro reports at the Tire Rack help a lot when choosing rubber for your car...not to mention the expert advice of someone like Jim. Give him a call, check the Tire Rack site for their tech on tires section,both will set you straight.

The Chemist

 

These data from Tirerack:

Traction rating: Avid H4S: AA, Fuzion ZRi: A

Temperature rating: Avid H4S: A, Fuzion ZRi: A

Speed rating: Avid H4S: H, Fuzion ZRi: W

Avid H4S has the same temperature rating and higher traction rating, but lower speed rating than Fuzion ZRi. I do not understand how is that possible.

 

By the way, the traction rating is for wet pavement...which explains why some "cheaper" less performance oriented tires have higher traction ratings. Look at some of the competition tires, there are a couple that have C traction ratings (Hoosier AS305 autocross tires being one of them). Here is a quote from the tirerack site:

UTQG Traction Grades are based on the tire's straight line wet coefficient of traction as the tire skids across the specified test surfaces. The UTQG traction test does not evaluate dry braking, dry cornering, wet cornering, or high speed hydroplaning resistance.

Traction - coefficient of friction
Grades Asphalt g force Concreteg force
AA Above 0.54 0.41
A Above 0.47 0.35
B Above 0.38 0.26
C Less Than 0.38 0.26