[yetihehe]

I 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.

[birdsongs]

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.

[fy20]

Pretty normal with diesel as it gives off less heat than petrol. I have a van with an 88kW engine, and even at -5c I can see the coolant temperature drop when I am idling down hill and have the heater on. Any colder and it's worth blocking the radiator with cardboard.

[westmeal]

Protip for next time, cover the rad halfway with some cardboard and this will help a bit.