...perspective and context are important here, especially in re: wet weather, standing water, etc.
Look first at TR's review of Value-Priced Ultra Performance All-Seasons Tires on their new F30 328i
http://www.tirerack.com/tires/tests/chartDisplay.jsp?ttid=173Average stopping distance in the wet = ~115 ft, and the Best tire gets a 7.14 wet rating
Now here at TR's test of Grand Touring tires on the same car:
http://www.tirerack.com/tires/tests/chartDisplay.jsp?ttid=171Average wet stopping distance 108ft, and the best-rated tire gets a 7.19 wet rating
In the link of the TR's new Extreme Street Performance Tire test I linked above, the wet stopping distance for ALL tires is under 101ft, and the WORST rating is 7.46 (ZIIs) --still exceeding the GT and UHP A/S tires by a not insignificant margin. Even the Rivals outperform both classes of tire in the wet.
(As an aside, the data from the prior-generation XHP tires in re wet braking and wet skidpad are even better, but the car changed from the E92 to the F30...that may skew the results somewhat.
http://www.tirerack.com/tires/tests/chartDisplay.jsp?ttid=118)
There's more to wet grip than tread pattern---compounding still matters. Sure, channeling of water is very important but using TR's own data the Extreme Class of summer tires outperform the ones I would expect to have better wet weather traction/stability. (edit: maybe instead of saying XHP Tire "X" is "awful" in the wet, it should be looked at in the context of being "not quite as excellent" as the other XHP tires?)
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Steve Carter
1972 Datsun 240Z-- resto pics at
http://picasaweb.google.com/srcartermd2007 GPW Honda S2000-- STR 86