They are bulky and eat up a lot of space, but when you’re stuck in an unheated vehicle for any length of time they will be your Best Friend. I keep a military Modular Sleep System in the Box O’ Gear but it really doesn’t end there. Rolling around in the back of the vehicle is also one of the older style GI extreme cold weather sleeping bags. Both bags are bulky but since they are pretty much not going anywhere except in the vehicle, who cares? And, broadly speaking, bulk equals warmth.
The MSS is nice and very handy, but for some reason I really prefer the older GI extreme cold weather bag. Don’t know why. I’ve tested both bags in 0-5 degree weather by stripping down to shorts, t-shirt, and socks, climbing inside one of the bags, and trying them out. Both function well and while you may not be comfortable in the 0-degree weather, you will be warm enough to keep your toes and not die. I keep both bags in the vehicle because I can and I like to be prepared. Plus, if you get stuck with more than one person in your vehicle it would be nice to not have to listen to them complain. The MSS has one very nice feature…since it is a bag-within-a-bag system, it lends itself to summer emergency kits as well. Using just the patrol bag will be more comfortable in the summer than using the whole system. (But, obviously, keep the whole system together in the vehicle.)
I also keep a surplus wool blanket in the vehicle as well. It’s a distant choice for when you get stuck, but for those long roadtrips where someone is cold and want’s something less involved than unrolling a sleeping bag, it’s a good choice.
Both bags get rolled up and crammed into protective stuff sacks. If your vehicle goes sliding off the road at any speed there’s a good chance you’re going to come to rest, suddenly, against an object that’ll bust out a window or two. If that happens, you’ve got all sortsa weather suddenly coming into your vehicle and it’d be nice if your critical gear was protected. (Thats why the Pelican case.) Even just stuffing the rolled sleeping bag into a couple layers of heavy duty garbage bags and wrapping them in duct tape will do the trick.
You can use whatever sleeping bag you think works best. I went with the uber-bulky military bags because they’re cheap, durable, really warm, and I’m not worried about their portability. All that matters is you want the warmest sleeping bag you can come up with. Sure, maybe you’ll get too warm…big deal, open the zipper a little. Ten below zero, the rear window on your vehicle is shattered, you’re in the barrow pit, and the road is a skating rink….at times like that there is no such thing as a ‘too warm’ sleeping bag.
Years ago me and a buddy had to drive to Helena, a couple hours drive, in the middle of January, in his vehicle that did not have a working heater. I was amazed..astounded, really…at just how cold an unheated vehicle can get when you have to sit still in it for a couple hours. I had assumed that having the engine going would provide at least some level of warmth. Nope. And that was with the windows rolled up and us bundled up. No lie, man….it gets downright cold in a vehicle when it’s the long, dark, night of winter and there’s no heat.
As I said, I go for overkill. Two sleeping bags and a good heavy wool blanket. Do not carry just a blanket. Whatever you get, wrap them in some sort of protective material or container to keep them dry and clean. (As pointed out in comments, stuffing a seeping bag into a five-gallon bucket and sealing it up makes an outstanding sleeping bag protective container.) If you have to spend two days huddled in your sleeping bag in the back of your Subaru the last thing you want is that bag soaked in old Pepsi, motor oil, and any other fluid that exploded out of the containers you keep in the back of your vehicle.