That's cold indeed! Most of the places I have lived (and my current home of Bozeman, MT) also get down much colder than that. Still, a zero rated bag that's honestly rated would probably save your bacon if you were stranded in a car. First off you could put on any and all clothing you had with you, which presumably would include a winter coat, cap and gloves. Second, the sets of your car would insulate you from heat loss by conduction better than any ground pad would. Lastly, even without power or heat, the interior of your car should trap some heat and at least will keep you out of the wind.