From my racing experience, summer tires are still grippier in both the cold and the wet than all seasons down to around 6 or 7 deg C. Beyond that, I have no personal experience with whether summer or all seasons would be grippier, although it should be obvious that the colder it gets, the more advantageous all seasons will be over summer rubber.
Additionally, in warm and dry conditions, winter tires just fall flat on its face. Instead of being a summer = 10, all season = 9, winter = 8, it is more like summer = 10, all season = 8, winter = 5.
Also, in snowy/icy conditions, winter tires are vastly superior to all seasons la. Every tire test you can find will tell you exactly the same thing.
-Lik |