alt Hacker NewsI had to drive in -30C once, the engine could not get up to final temperature after 2 hours of highway driving because I had to run cabin heater at full blast on windshield and side windows so they didn't cover with fog inside. But that was in very old low power car.
My tiny diesel car (2008 Toyota) needs its auxiliary heater below around -15 C for highway trips. It's a switch in my dash that burns extra fuel, otherwise the engine won't get up to or stay at temperate.