[wetphoenix]

~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.

[ansel12]

A mummy bag isn't better insulated, it warms better through it's design and shape.

My army bag (U.S.) is very generously cut and it is one reason that I like it, and the super high quality zipper down the center makes it ideal for sitting in the car while the mummy shaped shoulders still remain in place and the hood is still usable while sitting up, or for escaping it's confines when sleeping on the ground.

I have camped at 30 below zero using that bag with blankets in it and it is very comfortable, I have no interest in normal bags that have the cold air pouring in around your shoulders and your head exposed.

It is so perfect for sitting up in a car that I used it to drive cross country during the winter once, in a car with no driver's side window or heater in it. I bought a second one for any car passenger that I might travel with during the winter.