Yesterday morning I got up at 5AM to plug in my Insight. I have a 1000w electric heater for the cabin, 12v charger and 400w block heater all tied to one plug in the front.
At 8AM I got on the road toward my father's, with the outside temperature at -10F. The Insight started using its backup 12v starter. The clutch and transmission felt like they were full of jelly.
The roads were not great.
However, after around 3 miles the coolant was fully up to temperature and I was able to stay in lean burn as much as I wanted. Although I can easily tell based on how the car feels and what the FCD says, a 0v reading from the secondary O2 sensor confirms lean burn.
My simple WAI plus full grille block allowed for intake temperatures around 80 degrees above outside air temperature.
After 27 miles and around 50 minutes of driving, I arrived at my father's place a bit higher up in the mountains, with 62.7mpg on the FCD. Temperatures at his house were a bit lower:
From there, I took him to his medical appointment. Despite the forecast to warm up a bit, temperatures never went above 0F for the entire day and, although coolant temperature leveled off after the first 5 minutes, it clearly took a lot longer to get the other fluids up to temperature. As the car continued to warm up, the FCD gradually crept up to a hair over 70mpg average before I had him back home. I imagine that, even at these temperatures, on an extended trip over level ground I could probably get up to nearly 80mpg.