I assume you haven't lived or traveled to cold places?
I'd say in the ballpark of -10°F is as low as it's gotten in southern NH.
As for me, I once spent a weekend skiing at Jay Peak in VT (on the Canadian border), during a "cold spell". It was -10°F at the base of the mountain, in the daytime. But the night we drove home, my trusty Mercedes 190E's external temperature reading hit -27°F.
I've traveled quite a bit, but it's always been during the warm weather months.