So, used owner here, and haven't owned it long enough to experience the mild California winter with the car yet. So, coldest I've ever experienced here at the bay is 30 F. That doesn't happen often. We're lucky to see 30 at all. At what point does the heater become useless? It it's only in the 30's, I'll just deal a couple days, but if it's 40's-50's, I may consider driving my 17mpg gas hog those days.