I would bias it towards being later putting on summers, rather than earlier. Before trading in my last car I ran it all summer on the winter tires, b/c I knew I was turning it in on them (Conti Wintercontacts). I was expecting it to just burn them right down, but they hardly showed wear. So now I wait a little longer, until confident it is above 40 (5 to you metrics) - the summer tires do chunk and crack if it is too cold, and real problems if you get caught out in a late snow, even if only a little dusting. I found a doctor who can induce one of those propofol comas for the next 2 weeks, that helps me with my impatience to get onto the summers
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