A big flannel sleeping bag is definitely comfortable, but just one would take up the whole area behind the back seat of a Wrangler! Mummy bags for backpacking such as this Ultra 20 model from GoLite are as small as a two-liter bottle, and in some instances they can keep you warmer than that huge sleeping bag. The mummy design has less void space that your body must heat. The tradeoff? A good down sleeping bag can cost significantly more than that big flannel bag.