Man I love my old mummy bag with the zipper down the front.
I put a couple of fleece blankets in it to make it warmer and easier for me to leave it party unzipped, while I eat and read at night when I stop the car to sleep behind the wheel, preferably in dark, snowy mountain passes, I love sleeping under those conditions in the winter.
That is truly one of my most pleasurable activities of winter traveling, the old roads are perfectly dark, no other cars, and the mountain passes get so mysterious and enchanting.
~Man I love my old mummy bag with the zipper down the front.~
I know that mummy bags by default has better insulation but the way they are arresting your movements is pretty much uncomfortable.
While you are traveling in a vehicle it’s size and weight is not so critical so I prefer heavier non-mummy bag for same performance.
Mine is SNSh-3 which is not recommended for use under -10 Celsius yet I’ve once quite comfortably lived a night in one at -28 to 32C in the open. Also had an Arctic class flight suit on, though.