Travelling from Nevada to Alaska in the Middle of Winter

^True story. But mostly on the downhill stretches. Down to -25F I’ve never had issue with a Dmax or Cummins making full temperature, if towing and highway speeds.
But it will help with warmup and even more so on long downhill grades.

At -20 or colder I don’t think you be able to keep the temp up coasting downhill without one.