The general consensus between tires manufacturers is that standard tires get too hard below 45degrees and don't offer the same level of traction that they do above that temperature. So, it doesn't have to be -30c to need winter tires. And if you happen to have extreme summers like Pilot Super Sports, they have big warnings on them to avoid driving in temperatures below 45degrees for that reason.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i